What to say to clingy class guy?

Depends on how he acts when he's bothering you.

If he asks you out, you owe him a straight up "no."

If he hasn't directly made a move, you will have to be direct about exactly what he's doing. If you just dislike him and don't ever want to hear from him again, you're going to have to tell him that. However, it sounds like you just want him to give you space and quit trying to get to know you better. "Dude, what?" or "make it quick because I have things I want to do," or "you're not actually hitting on me I hope" might be the kind of responses you're going to have to learn.

I should also add that you'll need to be direct about what you want (less attention, more distance) before asking someone like "big brother" to scare him off. It's fine to have a big brother-type there when you do it, as long as you're not focused on embarrassing him or making anyone laugh. If you're smart, you'll do it without being abusive. Just do it, though, so that you're not wasting each other's time.

For both your sake and his just tell him he's not your type.

He must be really persistent. Just lay it out to him that he is not your type. You don't have to shout at him "leave me alone" but just say it straight. You don't like him, you don't feel anything for him, and for his sake, please don't tell him "let's just be friends".

Why can't you just say "LEAVE ME ALONE"? makes no sense just tell him to back off be honest.

you have to lay it right out for him, a lot of guys don't catch on to hints. You may not want to hurt his feelings or make things awkward, but unfortunately if he's not taking the hint then you won't be able to get around the awkwardness, and depending on how much he likes you, no matter how you say it his feelings can get hurt. you just have to do what is best for you and make your feelings clear.