Hitting should never be an option unless it's self defense. If people are injured in a fight, no matter why, the one who swung first is responsible. Walking away is always best. Bullies are idiots who need to feel better by making others feel bad. They are low lifes and usually quite ignorant. And sadly, many are bullied at home and do it to feel stronger. They thrive on reaction. Don't give them any. You're better than they are. Ignore the bug and it leaves you alone.
Hm this is kinda hard to answer well I guess if he / she starts to get too violent it is best to stand up for yourself. Show the person that you're not afraid give the person a taste of his own medicine or if don't feel comfortable try to find the time to talk to the princepal.