If they are tearing down your fence and endangering your animals, the police have to make note of the complaint. Unfortunately there won't be much they can do unless they catch someone on the property, but it is the first step and you need to do it. And you really need to put up no trespassing signs. I know you said they blow down, but there has to be a different way you can post them so they would stay up. I would also absolutely FILL the path with brush, logs, anything else you can find that will prevent them from driving right up the same path again. I wish you luck.
"Keep a leg on each side and your mind in the middle"