This is a very, very difficult thing to train. I have had guard dog's beaten by poisoning more than once. The tactic is very effective! All the intruder has to do, is throw some tainted meat into the dogs yard and then sit back and wait. It is very difficult to train this food instinct out of a dog. The only effective way for it to be done is train your dog to only accept food from you and you alone. Which means nobody else can ever feed your dog, you can see why it is so difficult to accomplish this. As soon as you have somebody else feed your dog it's over.