Read carefully through this.
When you put someone on Ignore it will ignore every character on that account for your character. BUT you have to do it from each one of your characters separately.
Never ever take them off ignore and never reply to them. When they switch accounts to circumvent Ignore, you put in an ongoing harassment ticket.
It takes time, but Blizzard will deal with them when it is shown that you have used the tools on your end and they won’t leave you alone.
That said, harassment means contacting you via chat and such. It does not cover PvP, being in the same physical space as you, fishing where you might be, killing mobs near you, farming resources near you. I know you did not say they were, but I just wanted to make sure that part was clear.