I'm not too sure about the suggestions of take up martial arts, go and punch 'em as hard as you can, that'll show em! although I'm sure it has it's merits. However there's been a few instances of the advised 'single punch' turning into a battering. What happens when there isn't a semi responsible adult to stop things before they go to far? Or as Colin suggested, these days they might have just come and stabbed him.
With regards to physical violence I have two rules:
Nobody has the right hit anybody else.
Everybody has the right to hit back.
It's fair enough on one hand but on the other it can easily perpetuate. How about more diplomatic methods, or shame and humiliation that doesn't involve fists?