If someone is a Bully its usually one of two reasons
1. They have a bad life at home
2. They are attention seekers
If No. 1 you can blackmail them, if No. 2, thats the more violent bit.
Please Note, the MOST painful and MOST immobilising place to hit a male, is the balls.
EDIT: The person will automatically groan and lean forward if they are kicked in the balls, if you are feeling brave you follow it up with a knee to the face.