Beat the crap out of the bully. . . . :D
Seriously though, bullying is often a result of children not being taught to respect those around them. The real way to prevent bullying is to educate everyone to treat each other with dignity and decency. It would have to be a complete effort from all of humanity.
This isn't a real solution at the moment. The human race needs to become more sane for it to happen.
On a personal basis, if a child is being bullied, teach them how to defend themselves physically and verbally while still respecting their fellow person.
If you are trying to get a bully to stop, find out what's going on with them, then teach the individual about respecting others.