He's getting attention with it (he doesn't care if it is negative) so it is going to continue til he decides a more appropriate way to get attention is just as fun.
Also, have you had him take martial arts classes like Chris said? Right away there, the kids learn they are not permitted to fool around with their karate chop moves!
My son doesn't do karate chop moves, but he does flail around (being silly, playing) and has a limited concept of personal space. So he will step on kids or their lunchboxes, hit them in the face, or knock them over. He has gotten put in detention for these behaviors even though they were not at all malicious. The end result of his behavior was another child getting bonked, though, so he had to face the consequences.
We have worked with an OT and with social skills groups to get him to improve his awareness of personal space for his own body as well as respect for other people's bodies. I'm not sure how much those have helped as much as him getting older. Probably a combination.
Kate
