I will second this. I had a few people bullying me in highschool, but never resulting in any sort of violence, just namecalling/etc. One day, when I was hanging out with friends, he came over and started calling me names and generally being very confrontational. At that moment, I realised that I had to put an end to it then and there.

I punched him twice in the face, and he went down. When he got up, dazed, his friend split us up. He never even talked to me after that, and always avoided me.

If my kids ever get bullied, I'm teaching them Krav Maga.