from this it sounds as if the user is dbo of the database (not quite the same thins as being in the db_owner role, but it will have the same rights plus the ability to add other users to the db_owner role).

To remove this user you need to set another login as dbo of the database, use command alter authorization or exec sp_changedbowner for this.

running sp_helpuser you should see dbo mapped to the login.