I seem to have a Mr. Collins on my hands.
When I first met this Collins fellow I had a bit of a thing for him, but the more I’ve seen of him, the more I’ve realized that he is not the guy for me at all. He is constantly showing off, and he keeps trying to talk to me during class (like, while the teacher is talking!), which is embarrassing. Collins’ personality is constantly “on,” if that makes any sense, and it’s just gotten incredibly irritating. I’ve tried to withdraw myself from the situation, but it’s difficult because we have the same classes.
The problem is that no matter how many cold shoulders I give him and how many of his jokes I purposefully ignore, he cannot take a hint; he just tries harder the next to time to get my attention. I can understand that it may be confusing for Collins because I was initially interested in him, but that was months ago. The whole thing’s gotten to a point where I dread going to class because he’s going to be there, and it’s embarrassing because his attentions make it appear to everyone else that we are a “thing.” Jono, can you please tell me what is going on in his infuriating guy-brain? If he doesn't stop annoying me soon, I’m afraid I’m going to randomly yell at him, “I don’t like you!” and that would be awkward.
Well, you kind of already know what's going on in his guy brain: it is the brain of a guy, and therefore its prime directive is "engage in makeouts immediately, to the exclusion of everything else," with a distant second being "secure food and shelter to remain alive." For the record, most adult guys who have been around the block a few times will understand when you're trying to show that you're no longer interested. This guy isn't too savvy yet, and he's thinking "Well, she used to like me, so I guess I should just be myself, except more and worse, until she likes me again!"
Before I launch into some indirect ways to show you're not interested, I'm obligated to say that just telling him so—directly but politely—is usually the best course here. I know nobody ever wants to hear that advice, but remember that he has the brain of a dude, and is therefore prone to dramatically misunderstanding your body language. He may see your cold shoulder and think "Guess I'd better tell her an even louder awful joke! Why is this teacher yelling at me. Where even am I." So something more than just ignoring his advances may be called for. Here are some suggestions—hopefully the subtle ones will work before you have to be blunt with him.
DEFCON 3: This Boy Is Getting Pretty Annoying
This is where you are right now. At this point, I'd advise polite signs of disinterest (along with not responding to his advances). After all, he does have a non-insane reason to believe that you are interested (which is that you once were). So don't treat him like a gross alien, but do show exasperation instead of just pretending you don't notice his dumb antics. He needs to get that you don't find this stuff amusing, and that he isn't your type. This would be easier if you two were conversing about stuff, because you could subtly point out all of the ways you're not right for each other by highlighting your different life goals, politics, hobbies, and so on. ("Oh, you like football and action movies? I think football is the worst of the balls, and I would rather watch grass grow, at a cemetery that is exclusively for puppies.") Either way, the point is to make it clear that you're not just ignoring him, you're actually the wrong girl for him.
DEFCON 2: The Annoyingness of This Boy Poses a Clear Threat to National Security
If all of your politeness fails, then ditch the politeness. Start using direct rebukes and unmistakeable signs that you don't approve of these shenanigans. Frown at him, sigh heavily, stare daggers and/or icily. Stare ice daggers. Be blunt with him. I can understand a truly dense boy misunderstanding signs of vague exasperation, but literal babies can understand what an angry frown means, at an age when they are so dense they will repeatedly try to eat a plastic hamburger. I can't picture any dude making a person visibly angry and still thinking that he should just flirt even harder for some reason.
DEFCON 1: If I Had an Army I Would Mobilize It To Deal With This Boy
Look, I realize that you don't want to actually have a conversation with Collins about this. But if even rude dismissal doesn't work, then we're at the end of the line. Two men in dark suits are unlocking a panel with two matching keys. They are revealing a giant red button that says "JUST TALK TO THIS MORON ALREADY." At this point, you really have to take him aside and explain—as if to a literal baby—that you're not interested.
I still urge you to explain the situation in a polite way, because even shouty goofballs don't deserve to have their hearts broken. Find a time to talk to him and say something like "Look, I think I gave you the wrong impression. Sorry, but I'm not interested in you in that way." Then stop, and let him respond—if you're really exasperated by this point, your pent-up feelings might come out in the form of an unending run-on sentence: "...and, AND, I hate your jokes AND your talking in class and your FACE and your TERRIBLENESS and ARRGH." Just calmly make it clear that his interest is unwelcome and you want him to knock it off. This should work, because it always works. If it doesn't, then you're going to have to get your advice from Batman or somebody, because you're dealing with a person who has elevated "not taking the hint" into a bona fide super power.