I have neighbor that is also pretty much a starfish but isn't even close to as bad as what the OP has had to go through. When we painted a side of a stone wall white iirc (their wall, but on our side) a few years ago, they loving repainted over with black, which would have required them to be on our property. Sure we shouldn't have done that, but then again it wasn't on their side and they didn't have to be a ass enough to paint it over themselves when we aren't looking. Then, we put up a fence to be a easy solution, and they hired a lawyer to attempt to claim the fence was crooked in what was apparently an attempt to get rid of it. After that failed, they pretty much gave up but they are still bitter towards us to this day.
Anyways, that sucks far worse in comparison considering they have and are indirectly trying to rob your family out of thousands of dollars (and potential money lost due to people not wanting to live next to those people). If I were you, I'd fight fire with fire in a legal way of course; buy firecrackers or something and explode them at 2 AM when they are trying to sleep or blast a air horn multiple times in the air not directly aimed at their house at 2 AM. Unless they are recording 24/7 waiting for you to go over the edge to forget you over even more, it would have never happened unless they are really that convincing to their corrupt cop buddies.