That’s actually the first mistake. Making contact with this individual makes it a conversation and not harassment. Everyone needs to put this person on ignore. Nobody should be trying to tell them what they can and can’t do.
If people would stop interacting with him, perhaps he’d move on. But if he starts bypassing people who have him on ignore, a ticket for ongoing harassment can be put in.
But it requires that everyone simply ignore the person. No replying to them, nothing.
Here’s a detailed write-up from one of the Blues here that you might want to share with your guild or at least the guild leader: