Video camara. That is what I would do. And document, document, document. Then take the tapes to the police. Let the police help you with this. Perhaps these creeps have outstanding warrents, maybe parking tickets, maybe drug use, child welfare.
I think that most creeps like this have other issues that can be addressed by the police so let them handle it.
I would make them so uncomforable that they have to move. Several police visits usually do the trick.