As the other posters have said, I would have a talk with the owners. If they ignore your concerns, I would call animal control until they were tired of hearing from you. Your property is your property. They have no right to let their dog roam in your yard. They may think their dog is harmless, however, it's their dog and they should keep it on their property. Who knows what could happen. Stand your ground! Don't let them bully you just because they have lived there longer than most and think they own everything around them. I have had neighbors in the past just like this and I did not back down. I hope the outcome is positive. I will keep my fingers crossed!