Acting like that is reprehensible, for sure. Blizzard does not allow ongoing harassment but it can take a while to prove and you need to take certain steps to do it.
Here is the Blue post on it that goes into the details.
It is critical that you, and everyone else, take the steps needed to ensure he can’t contact you. That means Ignore and not engaging in any dialogue. When they circumvent Ignore to still bother you then you can log the name, server, channel, time, nature of offense. That can all go in the ongoing harassment report via the website.