Hmm, so far we've got several proposed solutions.
Run away/advoidance of them
Ignoring them
Retaliate against them
Preemptive strikes
Ignoring them avoiding them, and running away don't solve anything, personally, I consider it nearly irresponsible. If you do somehow divert their attention from you they will simply attack other targets. I accept that many are not capable of self defence though, so in that case disregard the bit about irresponsibility. Not everyone can be strong and/or cunning.
Retaliation is the one I'm most for atm. I mainly brought up only physical earlier but there're is more than one way to retaliate. Seeking someone with authority is a path to take but rarely brings results in a school setting, with proof its a more effective choice in other settings. There is also the route of manipulation, what you would have to do for this to work depends highly on the situation. Could be anything from turning people friendly toward them hostile to provoking an attack from them in front of someone who wont tolerate them. I generally feel bad about the manipulation approach.
Preemptive strikes drifts to close to defensive bullying for me. Reminds me of an inmate shanking the biggest guy they see in jail on their first day to establish dominance or something lol. I have no experience with this one so I can't really comment on it. I will say however that as long as its known, proven bullys I can't see much morally wrong in it.