Besides training the dog, your other option is to use a repellent spray. Liquid Fence sells a good, natural repellent for dogs and cats. It smells pretty bad while it's wet, but once it dries it has no real odor for humans, but it will keep cats and most dogs away. Dogs are not 100% controlled with this since each dog has a very different point of view about what smells bad, but it's worth a try. You can buy this product at most garden centers or home stores.