Most people don't have the luxury of being bigger then the bullies so that will maybe work in a small percentage of cases.
Hitting back isnt really a good idea either, because yes its giving them what they want, they're after a fight most of the time, and to engage in a fight is just as bad as ignoring it.
In my experience and from what i've read and been told it normally sterms to them being quite troubled people themselves, and although we love to hate them, they are in fact as much a victim to some degree and its their way of calling for help, unfortunately picking on others isnt normally read as a sign of calling for help.
I'll admit its not always the case but the best you can do is inform people and hope the people in the know can offer the kid help, because there's more then one person suffering, firstly the victim who's likely to be more scarred for life or seriously depressed etc etc..... the bully is also in need of help, punishing them isnt necessarily helping them........
You need to know what you did was wrong first, or as some may put it ' moral standards', without these a punishment is a deterant and not a solution.
All the same i wouldnt leave someone to get their head kicked in.