Consider his perspective for a little while.

Most player misbehavior comes from playing around.

A lot of your complaints come from the fact that your player is not taking the game seriously. He is not taking the game seriously. The game is a thing of humor for him. I repeated that for a reason: He's enjoying himself. That's what we tend to do when we play.

Now, with that in mind, let's address the misbehavior:

Clowning

Clowning around is one of the main things that I've encountered as far as issues go. Fortunately, it's not malicious, nor is it particularly damaging. I had a huge problem with this in my first campaign, where I was being all dark, grim, and serious as a novice cyberpunk GM and my players were exploring the effects of their actions on the poor, hapless denizens of the world they were turning into a mess.

This is when they do stuff like buy nine thousand llamas to crash the economy. They're doing it because they can, but they're not ruining your game. My foremost advice when this occurs is to just let it happen. Your player is enjoying himself, and the other players will likely remember it fondly. If you must clamp down on it, be sure to do so with the consensus of other players, instead of just trying to end the "misbehavior" that everyone but you is enjoying.

Redirecting

I've often had players just not be satisfied with the way my campaigns go (and I've been on the participating side of this as well) and decide to intentionally mess with my plot points to redirect the campaign. Ninety percent of the time this has occurred in my games it's been either the result of good in-character roleplaying (for instance, demanding the best gear if the character being played is a greedy, self-serving jerk), or the result of a total lack of interest in the current direction of the campaign.

To deal with redirection, sometimes a soft approach is better. Give your players what they want. Remember that roleplaying is collaborative storytelling, and even though you're doing the lion's share of the writing and creative process you're still responsible for listening to others' inputs when appropriate. Never assume malice when curiosity is just as likely the driving force; I've had players try to kill important NPC's just to see how I'd react, and while it's annoying it's also a way to prove yourself as a good GM by reacting prudently and without exploding (though specifics depend on you and your style).

Trolling

Sometimes players troll you. They'll lock all the other players' characters in a bomb shelter, but reinterpret the meanings of the phrase to result in a deliberate invocation of the "chunky salsa" rule and make everyone re-roll their characters following some explosive goodness. This is usually the result of a player who's bored, discontent, or offended in some way and wants to make the campaign more "fun", perhaps at the expense of everyone else.

The important things to look at when dealing with a troll are rehabilitation or removal. Sometimes it's enough just to ask them to stop, politely. Don't place a "red line", either. It's a great way to get a player to quit in a blaze of glory having killed everyone else's characters. Other times you need to just ask a player to leave temporarily or permanently because their behavior is just so disruptive, but this should be a last resort following other remediation (i.e. the one-on-one chat). Manage trolls privately, not in front of the rest of the group.

A closing note:

One thing you may need to do is set guidelines for your player. If they're constantly playing the same character's mindset, but just remaking them according to different rules, you may need to set boundaries and restrictions on them. Consider very carefully that your player sees things differently than you do; if he doesn't know where things are going, he may very well be disruptive without knowing it.

Also remember that he may be playing the villain in lieu of your NPCs; I've seen this happen multiple times in my group because one player or another doesn't think that the core conflict is interesting, and injects another one to the group dynamic just to liven things up.

If you don't read anything else, read this: Get in your player's head and talk to him before taking any action, communicate your concerns and expectations, and be prepared to use in-game consequences or removal from the group only as a last resort.

Rules as written: the subtleties of "can see" you're reading into don't exist. Whether you "can see" something mechanically means "you have line of sight to it." This heavily implied by the Player's Handbook, and finally made explicit in the Rules Compendium's section of Line of Sight on p106:

A few powers do require a user to be able to see a creature to target it, however. For instance, a power might specify that it targets "one creature you can see." In other words, the creature must be within the user's line of sight.

So the Warlock's curse simply refers to those enemies to which you have line of sight. You have line of sight to someone simply if you can trace the corners of your square to the corners of their square, and that's it, so whether you're covering your eyes doesn't change anything. Since covering your eyes doesn't change your line of sight, the only RAW answer is: nice work, smart guy — you still have line of sight to all the other monsters.

The rules are built on the assumption your characters are generally trying to look around and be aware of their surroundings, and not pulling these tricks. If it doesn't make sense why you can't use a spyglass to affect who you can see and change your line of sight, it's because you've exited rules territory, and at that point the rules aren't expected to make sense.

The RAW way to eliminate line of sight is to manoeuvre around obstacles and eliminate it. So that's what a Warlock wants to do, if they want to get creative with limiting their Curse targets by vision.

How can this make sense or be explained in interpretation or story?

Before this, there's a big principle of D&D 4e to understand: it prioritises balance and fun mechanics above rules making total sense story-wise. Thus the mechanics do not bend or adjust to what makes sense simulation-wise: powers and features do what they say, and it's up to the story to make sense of that. There is no attempt to simulate things realistically, which is a major point of contrast to previous editions, and a contributor to D&D 4e dismantling the Omnipotent Wizardry Tier of classes.

So, given this 4e ethos, the axiom is that Warlocks can only curse the nearest enemy in line of sight (whether the Warlock's covering an eye or not). As BESW points out, this is a fun and interesting part of the Warlock's tactical decisions. It's up to the Warlock's player and their companions to make sense of why this is the case, which could be a pretty fun opportunity to flavour your Warlock or their magic.

• It may be the nature of the curse. It refuses to be cast any other way, or picks its own target.
• The Warlock may be capable of casting it another way, but have good reason not to. Doing so may lead to very, very bad juju. Doing so might be violating their pact.

How would I handle someone trying to do this?

How we'd handle it in our own games is a matter of opinion and style. Some would allow it (especially if they're a fan of rule-of-cool and it was cool). Others would ask the Warlock to put the telescope away. I'd probably do the latter, so as to not have the small headaches that might follow from wandering outside the rules into simulationist territory.

It is a fun feature for a Warlock to deal with (having played one myself), and truthfully, having read how BESW would approach this, I'd do as he does.