I like the 1x6 idea. Put it about a 1/2 inch on of those trailers with a little dirt over it. They flatten their tires once, they'll be gone. What can they do, tell you to pay for it when they were trespassing. They'll find a new track real quick. Plus, I don't know how many police are in your area, but if itsa low amount, the odds of them catching them while they are trespassing are going to be tough. That trail camera ideo would be great, but unless they are moving at a crawl, I don't think your going to get a good image.