I would make him feel like an idiot. I would look at him square in the eyes with a straight face and say something like "You are such an immature idiot." Then, I would turn around like no big deal and go back to what I was doing. I would only do it once and ignore him after that. It'll make him feel stupid that you are above him and you think he's so lame and insignificant that he doesn't even phase you.