I've known a few situations when it ended because the victim fought back with enough force to scare the bully.
In my teenage years the class bully went through victims, often getting them to join him against the next victim. Quite a few did because they just wanted it to stop. When it was my turn for the bully's attention I took it for a while and then fought back hard enough he was afraid I might seriously injure him, and then it stopped. I think it also made it clear to the others who had joined him that they could be in significant danger if they picked the wrong target, so fighting back so hard also had the added effect of taking away any strength in numbers he originally had.