Cameras are a good thing, problem is you will need infra-red light for night shots. You also need a date time stamp on the video, don't forget to set it. Public areas are game all over the country and inside your home/properties.
A judge told me a long time ago that "Sometimes you have to take care of things on your own toughness".
I would first suggest you stop with the police and pay visit to the DA. You can press charges form trespassing to vandilism to stalking and making threating statements, such ordinances and laws are probably on the books and you can make the charge. Here, the police will sometimes, in situations where there is a lack evidence and one part insists on an arrest, the police may tell you to say "you're under arrest" to the guy and then they will take it from there. Doing so makes you liable and very exposed if you don't get a conviction. Other places, the DA will issue an arrest and file charges as you filing the complaint.
I do know someone in the business of curing problems such as yours. He is a martial arts instructor and past police officer, about 6' 2" and 240 lbs. If there is an issue he needs assistnace with he usually gets some of the local linemen from our university football team.
No, the don't break legs, they do nothing but talk to someone, but it is very effective. It kinda goes along the lines of telling the trouble maker to make sure that nothing harms the client or his property, that if so much as a beer can is thrown in his yard, he will be responsible and they will be back to visit. That usually does it or the bad guy leaves town.. Obviously hiring intelligent musel really depends on who is the problem and where you are, Philly might be different (LOL).
But I can assure you of one thing, the actions of these guys show that they are cowards. They act as a crowd and they get their courage from others. He doesn't confront you, he sneaks around to hurt you. Confronting the guy, with extreme prejudice, individually and in public might make an impression on him and drive him off. That's what this "security guy" does, It works with someone who can be intimidating, someone else you know that could go to him. He never makes any personal threats. It's to clear the air, to tell the bad guy it'sknown to be him or his friends, same thing, and that a response next time will have consequences for him. He makes them personally responsible for anything that goes on in the future. (gee how do you avoid that?) Your bad guy will deny it with every breath, make it clear his denial is not accepted. I guarantee you he is a coward.
I'm not suggesting you go jump on the guy'schest or get in his face, just telling you kinda what that judge told me long ago.
There is no way I would not buy a property because of one guy, maybe because of gang activity in an area, but not one guy and punk friends with one house. That's all for my rant! LOL