I wonder if this:
1) I banned a user.
2) I removed that user from the database.
3) I removed that users posts from the forum.
Should have gone in this order:
1) I banned a user.
2) I removed that users posts from the forum.
3) I removed that user from the database.
Because when you deleted the user all of their posts got assigned to user number #1 (the built in place holder)
Then when you deleted all posts it would remove every single post that's been assigned to user number #1.
Or... perhaps not. I have never done these steps so I don't know for sure.
You said IIP can still see/access the forum? There may be something ubbthreads.php looks for specificly that IIP doesn't when displaying the information. It may not be a corrupted table. (and I wouldn't expect it to be)
