As for in game: Open a ticket for ongoing harassment, not for “just” harassment. It is dealt with differently. Specifically when you ignore someone and they make new characters to continue harassing you.
As for outside the game: Get the police involved.
https://us.forums.blizzard.com/en/wow/t/spotlight-on-harassment/10309
Also: Don’t reply. If you reply or snap back Blizzard will NOT always be able to view it as harassment. If you don’t react, they will. It’s also in the thread I linked I believe:
“Remember that you should try to avoid contacting and/or replying to the offending player(s); simply keep reporting the harassment and allow us to investigate the matter for you.”