Just kick em in the crotch or smack em up side their head. I'm only half joking. You can half kick em in the crotch. So just when you're about to land the hit, pull back a few inches. The smacking them in the back of the neck when they're not looking or even in the face might actually not be too over the top. Just don't over do it that it causes serious injury. So like rough play. Seriously though, you only have about 2 options. Walk away and never look/speak to them again or threaten/commit non-life threatening violence. Both of these are consequences you can levy against someone when they step over your bottom line. I guess another option that may be available to you is to have another person enact the punishment. Say for example, tell your principal or teacher or some higher authority figure. For some people, words fall on deaf ears, even if you were to say them. And walking away won't deter them from harassing you. If I were you, I'd find new friends and also start getting bigger in the gym.