Any direct intervention on your part may backfire. Bullies become even more determined when adults get involved. Bullies can't be reasoned with. They tend to be very troubled children with poor self-concepts, poor social skills, and tumultuous family situations. Most of them are starved for affection and acceptance.
Bullies do understand force.
If all else fails, consider asking law-enforcement authorities to get involved. The bullying should be stopped as quickly as possible. The longer it continues, the longer it will take for the victim's emotional scars to heal. Also, some target children eventually begin expressing their anger toward younger and smaller children. Here are three examples of how parents have successfully handled a situation heading out of control.