1) Wait until they appear again behind the door. 2) Upon them entering your property open door and confront them about trespass. 3) Take pics of the more persistent offenders and produce as evidence to the local "Wannabee" Police (PCOs) and local council to deal with as nuisance.
4) More drastic action...................Obtain a good shotgun with 30 shot cartridges (Or blanks) and launch a volley over their heads.............although it *might* attract the wrong sort of Police presence ;-)