So you stop someone who is verbally bullying you by physically bullying him? I guess that's how the world works.
Well if the person being bullied could stand up for themselves in the first place then they wouldn't be bullied. So if you are being bullied find outside help.
A few years back I had a guy picking on me (tripping, pushing, general harrassment) and I just kind of took it for several months. Eventually I snapped, grabbed him by the throat and threw him against the wall. He punched me in the face (glancing blow) just as a supervisor was walking by, so we both got in trouble. We came to terms during our punishment, and it was all behind us. Moral is, violence solved the whole thing.