You just have to tell them off and stand up to them. If they confront you, you need to be the one who hits first. Ram your forehead into the guy's nose, if your lucky and you break it he will never fuck with you again. I've never done this, but I know that when you break a person's nose they don't mess with you. And, if you bash your head against his then people think you're hardcore. They won't bother you again.