I agree with @Marie405, take your vacuum and cleaning supplies in hand, let yourself into the space (with a friend and a cell phone) and let them know you need to start cleaning because you have an other guest arriving. If they are hanging around because they are waiting on a flight or a friend or a drive, point them in the direction of the nearest cafe where they can wait in comfort. MOst likely, that will get them to shift. But if they are still refusing to leave, remind them that they only rented the space until xx o`clock, that they are currently on the property without your permission, and that you would hate to have to involve the police, but are prepared to do so if necessary.