From what you say, these guys are not only committing theft, but also damage to property, and attempted bodily harm. I would definately get the law involved. You need to inform the landowner of what is going on. He needs to permanently ban them from his property. It sounds like this could get ugly. Thankfully I don't have to put up with crap like that and have never had to. I would also say put cameras out to catch them in the act, but they would probably just steal them if they knew they were there. You had better hide them good and make sure they have no flash.