What a Moron!
I really do feel for you! I would have to say that spraying your trees with round up (or any herbicide) and/or burning them amounts to vandalism and charges can be pressed and restitution accounted for. Of course that could also backfire and create more problems. Usually, if a neighbor's tree grows limbs over on your property, you have the right to trim them away (which is what he should be doing), but you don't have the right (or legal authority; at least here in AZ) to destroy someone else's property on their land. I would talk to the local authorities about that one and get evidence (trail cams are great for that) to support your claims. As for the donkeys, well, I agree, ignore the SOB and hope he moves on once his (apparent) one brain cell and associated synapse starts firing and he figures out he doesn't matter to you. I hate to compare a moron like that to my kids (5 yrs and 21 mos), but if you ignore the tantrums and unwanted behavior they will lose interest and move on. The other piece of advice would be this: "Never have a battle of wits with an idiot. They just bring you down to their level and beat you with experience!" Good luck!