A bully

I’ve know about this person who bullies people all the time online and face to face.  Let’s call the person X.  X has a habit of gathering people around to attack the victim and make fun of them.  I have witnessed numerous incidents of online bullying, and confronted X several times about this behaviour. People have told me about all the awful things X has done to them.  People talk about it but feel too frightened to say anything against X publicly.  Some people have reported the issue but things have not changed.  The bullying continues.