5
I know it goes without saying, but first make your back up.
If you have phpMyAdmin, then you should be able to select the tables, and then at the bottom should be a drop down box, in there are options such as 'Check Tables', 'Repair Tables' and 'Analyse Tables'.
You can repair them from there if required.
If your going to remove the user as Catz suggested, then if possible get the UID, normally in the url of their profile.
Or if you know their username (which I think you said you did), then you can always do a search on the users table.
_users
Hope this is of some use.