Is a signed statment of promise to vacate enough?

I have friends staying with me. I am the homeowner and they have been staying with me (family all sleeps in the living room) for almost a year! I finally gave them a written 30 day notice to vacate and they agreed and we all signed it. Now Im afraid they are just thinking I will let it go...I have for too long already. So if the date comes and they are still just sitting in my house and I call the police will they MAKE THEM LEAVE if I show them our written agreement?