Can't you just rename the user? If you can't do it through the admin section, you can do it in the database without messing anything up. In the `qa_users` table, edit the `handle` field.
The alternative is to delete that row from the user table - all that will happen is any questions/answers they posted will show up as "by anonymous".
Update: you can now delete users through the admin interface. Just go to the profile and click "Delete user".
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