Tell him it hurts, and that you would find it much more attractive if he didn't do that. You don't have to find him attractive, or pay attention to him or anything, but if he likes you and does it because he likes you. Then he will probably stop once he knows it's not attractive. Now if he doesn't like you, I wouldn't give him any sort of pain, because if he gets off on giving pain, he probably gets off on receiving pain as well. I'm normally not into giving incentives, but maybe tell him, you will let your body go and get out of shape if he keeps doing that. So that way if he stops, he will still be able to admire your beauty. Whereas if he keeps on doing it, he will think you will let yourself go, and therefore think you will become unattractive, which should ultimately make him to stop.