This question was prompted by reading this answer.
That answer, and accompanying comments, indicate that a Warlock can curse the closest enemy that he can see, even if he's aware of enemies that are closer. This seems correct and reasonable, based on the first sentence of the Warlock's Curse description in the Player's Handbook:
Once per turn as a minor action, you can place a Warlock's Curse on the enemy nearest to you that you can see.
Positioning
Therefore, if a Warlock wants to curse an enemy that is not nearest to them, they can fix this with positioning. They can simply move closer to the enemy they want to curse of course, or they could move so that they can't see the closer enemies. Seems simple enough, and it doesn't break the restriction because it encourages and rewards tactical gameplay.
Roleplaying
That's not what I'm curious about though. Let's say I'm a warlock character. If I close my eyes, I'm blind and I can't see anybody. Ergo, I cannot curse anybody. Okay. What if I get out a spyglass and look directly at somebody across the battlefield. There are 10 enemies closer to me than the one I'm looking at, but that's the only enemy I can see. So can I curse that guy?
I'm thinking maybe not, since in 4E characters don't have facing and are assumed to be looking in all directions at once, and if that's the case how can I say that I'm looking directly at someone?
But I'm also thinking maybe so, because specific beats general and I can specifically say I'm looking in only this direction.
Conclusion
First and foremost, I'd be interested in a rules-as-written interpretation of this situation. Beyond that, rules-as-intended discussions and how one would handle this situation practically if one of your players tried it would be useful as well. Note that, if this is allowed, the proximity restriction completely disappears and you can curse whomever you feel like, just so long as you can see them.
Best Answer
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:
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.
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.