Just give them a slap with your hand as you go by. I think most of them will stop after they kick your ass for touching them.

People question why they do it, its nobody's business how someone takes a piss. Perhaps you should stop staring at people's asses when they're in the bathroom. And to get so upset about it, maybe jealously, maybe frustration because you want to dive your tongue in his ass? Quit checking out guys asses, and you won't have a problem. Or do what women do and wait for the shitter to open up and you can close the door and sit down.