Nothing should ever be deleted from any sort of issue/task/bug tracking system.
AFAIK, there is no option to delete a user, even for the admins. The best you can do it set the password for the user you don't want to some long, random string and then forget about it.
Disclaimer: The forums admin/mods are a separate team to those who maintain the Bug Tracker. I make this post on behalf of myself, not in my capacity at a moderator as I can no represent the sysadmin team.
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