Well, that used to happen quite a bit back when I was in like, say...middle school or something. However, one day I got sick of it, and I... well, snapped.
Ever since then, I've pretty much had the respect of the people around here. I respect them, and they respect me, it's all good. Now, there are a small handful who just don't get it. But here, let me give you some advice, well, it's how I take care of this kinda stuff anyway --
Talk to him/her. Reason with them, now, 90% of the time, this never works, but if it does, it'll save you some trouble. However, if it doesn't work, punch him. Hard. Just give him or her (if you're a girl) a good punch in the cheekbone with your good arm, the follow it with an uppercut to the jaw with your weak arm.
If they get back up, beat 'em. Even if you get beat up, you still showed that you were going to stand up for yourself, and that you aren't gonna take it anymore.