Thats interesting. I do believe that you can ask the person or people, who are not stated on the lease, to vacate the premises. Check your local rental legislation. Further, any one completing a legal document in a false name, may be committing an act of fraud, and the police may be interested. May be a good time to have a Property Manager looking after it for you. Sadly, we have found that tenants listed on the "Bad Tenants Databases" then go for the privately rented properties. Hope all works out well for you.