Erik Schmidt's answer is probably the better one to go with (as it'll help you find the root cause), but I'll contribute a bit based on what I see from your description.

From your description, you have a player who enjoys:

• Building and optimizing new characters.

• Participating in combat.

And who doesn't enjoy:

• Long-winded intrigue.

And yet, this player is playing in a campaign that is low combat, high intrigue, and where the penalties for building a new character are incredibly prohibitive.

When players are marginalized, they tend to try to get into the spotlight in any way that they can. And that leads to disruptive behavior.

This leaves you with a few choices:

• Try to get the player into a campaign that they'd enjoy (either by running an additional campaign, or by encouraging them to find/create a new group).

• Remove the player from your campaign.

• Find ways to compromise, attempting to bring your campaign and the problem player closer together.

The important thing to realize is that there is no way to force the player to like playing the way you and your other friends are playing.

Character Jumping

Some people like to play the same character, and enjoy the process of growing and developing that character. Other players prefer the creation process, authoring new characters and mechanical combinations.

Dealing with the first sort of player is pretty easy. That's traditional, long-form writing. We see examples of it all the time in many different forms of fiction.

The second sort of player, the one you have now, is a bit harder to deal with. Here are a few things I've picked up to help deal with that:

• Avoid introductions

  Either introduce the character during downtime ("And so the party rested for six months, and met a new companion"), or use the "poof" method ("of course I've always been a catfolk fighter. Human wizard? Ridiculous."). You want to avoid having players stuck constantly in the untrustworthy rookie slot.

• Maintain player parity

  New characters should be (or quickly become) the equals of established characters. It's no fun being the one guy who doesn't have an artifact.

  The rule of thumb is that anything that's relevant frequently, whether it's powerful mechanically or narratively. A new character can probably be allowed to do without a one-time favor from a noble. But they should probably get written into a life-debt from the king of the current kingdom (or be given something else).

  If the new character doesn't start with this stuff, then the stated goal should be to catch them up quickly (within a few sessions).

  A level penalty is right out, unless it's a temporary one (or you're playing a game where levels aren't super important). For example, in my 7th Sea campaign, new characters start slightly below the lowest player. But all players are brought to the same experience total periodically.

• Let them know that they won't get some of the depth other characters do, but make an effort anyway

  From a DM's standpoint, the hardest thing about a character jumper is that you can't be sure of who they will be in the future. So if it takes you three months to author a storyline featuring that character, and they change characters every two, that creates an obvious conflict.

  Let the player know about this, and give them an opportunity to work with you. And make an effort to work their current character into the plot from time to time anyway, as a show of good faith.

Combat and Intrigue

It sounds like this player enjoys combat quite a bit. Most likely, it's a chance to see how the mechanics of their character work, and to show off a little bit. This is all fine, except when it stomps all over the intrigue that other players want.

• Give them combat

  The easy answer here, is to make sure you're providing combat. Don't make talking the ultimate super-power. Give him a mixture of fodder that he can easily crush, and challenging mechanical opponents.

  This is where the bit about parity and artifacts above becomes important. If their well-honed killing machine is constantly shown up by the senior characters with artifacts, the combat probably won't scratch their itch for them.

• Signal shifts to intrigue mode

  Provide clear signals to the group when a shift to intrigue mode is happening. Set intrigue in populated areas (cities, etc.) and combat outside of it. Have combat-based foes rush the group, snarling (or ambush them with a poisoned blade), while intrigue-based foes sit calmly on their thrones.

  If nothing else, the occasional reminder that "Baron VonEvil is pretty well connected. Are you sure you want to fireball his face?" can help quite a bit.

• Always have a backup plan

  This player is going to blow stuff up. As the person who's planning things, that should be a possibility that you consider. How do you keep the plot moving if the villain doesn't get a chance to monologue? What happens if the whole room is fireballed?

  The goal isn't to always have a way for the PCs to succeed per se, but simply for the story to always have a way forward. Perhaps that means that a scheme of the late baron crops up shortly after the heroes fail to find the documents, and now they have a fresh chance to investigate.

  Or perhaps the important documents were partially sheltered by the lockbox they were in. They've been badly charred, but there's enough there to make out a new location to investigate. Is it a trap? Of course it's a trap. They're going in blind.

Communication

Talk to your players. Not just about how you want them to behave, but about what you're trying to do, and how you're trying to shape the campaign to the individual players. Make it a dialogue, and solicit their feedback.

• I'm trying to add in some more interesting "boss fights." What did you think of that last one?

• I really liked the way you guys managed to outsmart Baron VonEvil. That's given me some interesting options for the next couple of adventures.

• My goal is to tell an epic story of your rise to power. I think we could use a bit more of the political stuff, I just need to work out how to write that.

• Etc.

Recommended Reading

There are two excellent sections in roleplaying books that I'd highly recommend for dealing with this problem, if you can get your hands on them.

The first is for the entire group. It's located in the 7th Sea Players' Guide, on page 238 under the heading Resisting the Story.

It's a couple pages long, so I won't reproduce the entire thing here. But the representative part of it is this:

Don't poison the PC pool. Don't act destructively just because "it's in your character," and don't force other players either to ignore your action or kill your character because it's in their characters. Know where the line is and make sure you -- and your Hero -- never cross it.

This doesn't mean that there shouldn't be any conflict within your party. On the contrary, some of the best and most rewarding roleplaying experiences come from moral or ethical disputes between Heroes. As long as you know where the limits are, and when engaging conflict becomes irreconcilable conflict, feel free to pursue arguments with your fellow Heroes.

The Rich Burlew essay Making the Tough Decisions is another valuable discussion of this principle.

The second is for the DM. It's located in the 4th Edition Dungeons & Dragons Dungeon Masters' Guide (pages 8 through 10). It's a breakdown of the different kinds of players, why they come to the table, and how a DM can target each one.

Summary

This is a lot of text, and only really brushes on a lot of important topics. The core assumption here is that you would rather play with this player, than necessarily play your campaign as it exists now.

The key is to give the problem player things that they enjoy. With good communication, a willingness to tune your campaign to the players, and a bit of compromise, the problem player may become a lot more tractable when their needs are met.

Short Version:

Maybe P is overwhelmed by bookkeeping and it's distracting him from situational awareness. Help him make a mechanically very simple character without fiddly bits or conditionals to keep track of, so he can focus on making good choices rather than having good bookkeeping. Invite the other players to support P with advice and by being good role models for the behaviour he's trying to cultivate.

Long form answer, with rambling and details.

Back in my very first RPG ever--and also my first time as a GM--I had a player whose poor choices got him repeatedly killed. Let's call him Q.

Q knew the rules and mechanics quite well, but had a very hard time applying them intelligently to whatever situation he found himself in (like forgetting to heal himself as a cleric). Even more than that, though, was his role-playing: he really really liked to role-play his characters, but that got him in trouble because when Q got deep into his character's internal motives the PC would lose common sense and perspective about the surrounding context of his actions.

It got bad. Really bad. Q's second character was killed by the party for betraying them (he had a conversation about his friends over tea with a "nice" lady). At that point I shared Making the Tough Decisions with the group. He studied it carefully, had intense discussions with me about it... and as a direct result his fourth character perished of untempered curiosity: the characterisation "very curious" overcame the common sense "half these items are cursed and my friends are begging me to stop," until the pile of treasure he was investigating yielded up a lethal curse.

After that session I took Q aside and we talked. He knew he had a problem, and he was trying to "get better," but he needed help. I'd noticed that all his PCs so far were mechanically complicated and required in-game bookkeeping: advanced casters and races with lots of conditional features and spell-like abilities to keep track of. So we hatched the simplest possible character build: nothing to keep track of. No "if you're flanking, X also happens," no spells, no per-day abilities. If his character sheet said he could do a thing, he could always do it.

We wound up with a kind of Indiana Jones flavoured skillmonkey (a rogue chassis with homebrew mods to replace things like sneak attack because tracking whether you can deal that extra damage was beyond what we wanted for the build). He wasn't optimised in the traditional sense--but since another PC in the party had straight levels in the NPC Expert class, that wasn't an issue in keeping him relevant in the group. Instead he was optimised for what Q needed: a simple no-bookkeeping character to let him focus on situational awareness and making good choices.

At the end of each session he'd hang back --along with any other players who wanted to-- and we'd reflect on the game: what worked, what didn't. We'd consult (and if necessary research) and come up with what to make sure we did again, and what we'd change next time. (I've since found that any game I run which has some form of this "reflect and plan" dynamic after every session is improved by it.)

In tandem with another player rising to the challenge and being a kind of "teach by example" role model, it worked. A year later Q was successfully running complicated wizard builds with great party dynamics and great depth of character. He was a real joy to work with, and all he needed was to wade in at the shallow end of the bookkeeping pool instead of jumping into the deepest part head-first.

nota bene: My players have tended to treat the group dynamic as one of table-level cooperation between friends. However much their characters may be rivals, at the table they collaborate to tell the best stories, and I'm also one of the collaborators. In groups where players and/or the GM act as rivals at the table level of things, I'm not sure how much my experience will be useful. It sounds like your whole group isn't really on the same page in terms of their desired gameplay experience, and communication isn't really strong. Working on improving the "friends at the table" level of things might help your game in a number of ways.