Some dirty data there apparently. You can try to delete the user from the ezuser-table in the database. But remember there's also a contentobject related to that user.
How did you screw up your database? This answer could help to provide a better solution...
Try emptying the trash. When you remove a user, the same thing happens to it as all other data, it is just stored in the trash. If you delete if rom the trash, or empty the trash, the item will be removed from the database.