My parents rented a house to a young couple. They signed a lease and lived there for about 3 months. The woman's dad came to visit and eventually stayed with them. My parents told them if he was living there they had to pay 50 more dollars a month. They said he was just visiting. They started to fight and tore up some of the house, also bring a pig into the house breaking the lease. My parents just asked them to get out so they did. However after they moved out I suppose they gave the key to the dad and he stayed. We went to turn off the lights we thought they left on and there he was. We told him he needed to get out that this was not his house. He said OK. The next day we went back and there is a note on the door saying I am sleeping don't bother me this is my residnece and you will have to evict me. The police said he is right. He never paid a dime, never was let in by us, and now he can stay for free until we can get him evicted. The house is pretty much empty I saw a few movie and couple of bags of his and that is it.
Is this possible?

Unfortunately yes, it is possible. You're going to have to give him a written notice that his tenancy is terminated and he has to leave. The notice period depends on the laws in your state but in most states you have to give him 30 days notice. If he hasn't moved out within that time then you have to file a lawsuit to evict him.

Tell us what state you're in and we can tell you the proper notice period.

Also, you can file a lawsuit against the couple for the costs of repairing any damage AND you can sue them for the rent during the time that the father is there.

In the state of Missouri

Hmm... this is interesting. Since there was a written lease between your parents and the young couple, and since their father moved in while they still lived there, technically they were subleasing to him. That means that the provisions of the original lease control the situation.

The first interesting thing is that Missouri law says that if the tenant subleases without the landlord's permission then the landlord can charge double the rent.

The second interesting thing is that since the father is subleasing your parents can give him a written Notice to Quit, telling him to pay all outstanding rent in full within 5 days or vacate the premises. If he doesn't vacate within 5 days then your parents can file the lawsuit for eviction. So because he is subleasing they can give him only 5 days notice instead of having to give him 30 days notice.

This is a very good site for more info in Missouri evictions: Missouri Termination for Nonpayment of Rent and Other Rent Rules | Nolo.com