Most likely he keeps doing it because he knows it is irritating the crap out of you. So if you just pretend you dont care and ignore you he should finally just stop because your not getting mad or anything. Trust me, I know from personal experience, I have a little sister.
Ignore him when he does it. Chances are he KNOWS this ticks you off, and that's why he continues to do it. I know it would be hard, because it is soooo irritating, but IGNORE him, and eventually he will give it up.