This exact setup happens all the time in movies, so let's examine how they handle it.
If they are the only character, then as a GM, completely tune the story to them. They shouldn't have to do hacking, or at best they have to shoot their way in to where the Russian hacker who already knows stuff is. James Bond doesn't use keyboards. Avoid his minimums except for once in a while for dramatic effect, or to introduce Bond Girls who have that skill set. As for addressing his strength - he's an expert shooter, but is always having to go places where he doesn't have a gun, or just has a pistol when others have machine guns, or has a machine gun when they have tanks. Or places where you can't just shoot the heck out of everyone (like a public casino) without having long term consequences.
If they are not the only character, the other characters are in danger. What do The Killer and Hard Boiled have in common? Lots of OTHER people who get killed. The chick who's along that's in danger. Or your buddy movies with one killer type and one intellectual (or even just not a killer, take Rush Hour) - the killer has to spend a lot of their time protecting/coaching/handling the less combat oriented person. One of the big risks of having a min-maxed PC in the group is the min-maxed bad guys the GM has to toss on, who can often terminate the non min-maxed PCs in a round. It becomes the combat monster PC's job to avoid that, or else the whole party dies and they say "new game, and be less of a goon this time please."
This is of course advice for in-play. You should try to head this off ahead of time by disallowing (GM)/forgoing (player) total min-maxing by choice of system or GM guidance. Because as you note it ends up being unsatisfying even in your maxed area.
I don't have much experience with play-by-email games, so these are probably less useful in that context, I'm afraid.
Don't be subtle.
For any of these strategies, tell the players straight up what you're doing and why. Solicit their opinions and ideas, and create space for feedback about the techniques once they're being used. This creates an atmosphere of trust and mutual accompaniment toward mutual goals, instead of a sense of manipulation and mistrust. The best social engineering is the kind where you enlist the targets as willing participants.
Start earlier.
Provide a time and space for the group to socialize, catch up on what happened that week, and get the giggles out before you try to do anything game-related. How much time this requires will depend on your group.
Provide transition cues.
Playing music during the pre-game socialization and then turning it off, or starting up game-specific music when it's time to settle down, can be very effective. Other techniques include having everyone wash their hands (no, really, it works!), moving from one room to another, saying a prayer, ringing a bell... you get the idea. Be creative and find something that suits the reality of your group.
Don't forget to transition out of the game at the end, too!
If necessary, impose gentle carrot-and-stick.
The nature of the reward and punishment will depend on the group, and this requires group buy-in or it'll seem arbitrary and mean-spirited. But if you can get it right, rewarding the group for staying on-task and gently reprimanding those who don't can be effective. But don't lay this down from on high as the GM; this needs to be implemented by the collective or they'll balk.
Remember that rewards and reprimands can be very simple: often simply having behavior called attention to provides the requisite pride or shame. Combine with the transition suggested above: it makes for clear demarcation regarding when the carrot/stick dynamic is or is not in effect.
Best Answer
Talk to your players and ensure that everyone is on The Same Page
Your players may be creating characters contrary to your setting because they may not realize that your style of GMing is not the same as another game they have played in. Making a Socialite Noble bard in a kick-in-the-door style of game, or a Lich Wizard in a Good aligned campaign are going to stick out like a sore thumb. So it's important that prior to having all of your players create their characters you give them one very important thing. Perspective.
Give them the perspective they need to create a character that fits in with the setting you're trying to create. Explain to them the primary characteristics or common traits you would expect to find in a PC in a game that you're playing, and once you smooth that out you can let them read between the lines and make their characters.
You can also ask the players to give you three or four different concepts for a character that they would like to play and choose one that fits your train of thought.
If all else fails and the players refuse to listen, you can politely set guidelines on what a PC can't play so that you ensure that you don't see it during the game. Tell them that playing an Spellcaster would get them burned at the stake because the people of your game world are deathly afraid of magic to the point of burning anything that even seems magical.