:( my stepson who is not on the lease with his mother and myself has his girlfriend living with us. They fight constantly and he has kicked her out many times but always brings her back. My question is how can we keep this girl out of our house?

Remove them both and be done with it... ask them to leave... they are not tenants unless they are on the lease which you indicate they are not... tell them you are changing the locks on X date and that they better have their belongings gone by that date or you will have them thrown away... chances are they will break up soon after they need to move on.

Remove both of them, as long as he is allowed to be a tenant or guest in the home, he can have someone over.