My best friend, in elementary school, was like this. She was always punching, pinching and kicking me. One day, I was swinging on her swing. It was a swing that went very high, as it was tied to a very tall tree branch. As I swung back toward her, she pulled the seat out from under me and I landed flat on my a**. I was so pissed, I started getting mind-blindness. I recalled the times she punched me for no reason or pinched me in class, or humiliated me in front of other people. I jumped up and tackled her. Needless to say, she limped away with a bloody nose, which also happened to have a large amount of dirt packed into it.
She never punched me again. I also had a friend in junior high, who loved pinching me during band. She scratched me with her really sharp fingernails too. She thought it was funny. One day, after about 6 months of putting up with it, I took a ruler and smacked her arm, so hard, she bruised and had a huge welt.
I don't know why I just didn't stop being friends with those girls. I hated what they were doing and it made me so angry. However, after I would finally retaliate, they would slowly drift away from me, onto their next victims.
Now that I am an adult, I don't really run into anyone who does that.
Tallgirl.