I haven’t an idea how to react to an adult bully. But the only method of dealing with bullies in school involves severe consequences, with the only two options I know as using police/court, or expelling the bully. Bear in mind that it is considerably easier for the school disciplinary system to deal with the victim who fights back than with the bully who is really to blame.
There is moderate improvement in schools addressing bullying, but this is no consolation for those victims who still get bullied, and then revictimized by the school.