Friend endlessly stalked and harassed by another player on many characters

[LibitIncarne]

Blacklist and report. Even if they delete characters they GMs would know what account they are using.
Also stop asking them to stop. Just blacklist, report, and don't bother engaging. They'll grow bored eventually or have their accounts banned.

Seriously, at this point ONE BLACKLISTING is enough unless the "stalker" has multiple service accounts that they're using to do this.