A bully is not happy unless he has someone to beat up on. The bully whose victim has learned to fight back will find a new person to beat up on.
I fear I know my former bully's next target. He's in for a rude awakening. A person cannot be a victim if s/he fights back and this person will fight back and fight dirty if provoked. I am right behind him/her, throwing as many punches as needed. As long as I am around, my former bully will not be allowed to bully again.