Parents of bullies, when did you realize your child was a bully and how did you react?

Not a bully but I know I got one of my old bullies hard when, on suggestion from one of my cousins to invite said bullies to my birthday party (7 total, only one showed) and proceeded to introduce the kid as "the shithead who beats me up every day" even called him that in front of his mom. I know at some point the kids mom had a discussion with my mom (I'll ask if she remembers how it went) and after about 20 minutes they up and left.

Never had a issue with him afterwards.