I thought I had some horrible neighbors in my time, but you've got me beat. I'm a very private person, minding my own business and expect the same from neighbors, etc. Like I said, I've got some horror stories, but nothing like you seem to be going through. I do feel your pain!
I once had a gate that I kept locked, not only on the latch side, but also on the hinge side. I took a log chain and wrapped it around the fence post and thru the gate on the hinge side and padlocked it. For someone to take it off the hinges they would of had to cut the lock, chain, or the post. Never did have a problem. I wrapped the chain about half way up from the bottom of the gate so they couldn't just cut the fence wire pull the chain over the top of the post and lift the gate off its hinges. Maybe you could do something similar.
BTW, weren't you the one that recently bought a truck, and posted your buying tactics with the dealers? That brought back a lot of memories for me too, as I use the same MO. I only got shafted one time on a new vehicle, and that was over 30 years ago. I learned real quick after that one.