I would suggest that you look at what is involved in taking up Bee Keeping. A few well placed hives could discourage trespassing, especially after you "warn" those neighbors that the only bees that are resistant to Colony Collapse Disorder and Africanized bees which tend to attack any intruder that gets too close, even loose dogs. Yeah, That is BS but a fear of bees is quite common and could provide some relief. BTW, Bee Keeping can be a very interesting hobby and honey can be a rather nice bit of a reward.

Another really obvious suggestion is some well placed cameras that are well hidden and capable of producing high quality images. Because at this point your only recourse is clear documentation of what you are dealing with.