For everyone using this stuff: I will stress
EXTREME Caution with deleting stuff manually in the Database and would instead encourage you to use built in tools for deleting stuff.
Particularly Users, Posts, Groups etc.... never delete them directly unless you know all the other stuff that needs to be updated as well. Check the admin scripts that delete users/posts etc... to see what other stuff needs to be updated as well.
If you delete the wrong stuff it can cause threads/forums or more to simply not appear properly, as JOINs can not match up.
So excercise EXTREME caution when doing SQL Queries directly in the Database, as they are usually unreversable and/or can take hours to diagnose/fix if stuff gets out of wack.