Poor doggie. They are very irresponsible for letting him wander . It has been my experience that once you call the by law you will start a neighbour feud for life. I would suggest waiting until you see the owners and the dog together outside and approach them to let them know your issue. You have a right to keep your yard dog free if that is what you choose. If they are not willing to work with you on the solution. Then a visit from the by law officer is called for. My mother always said . The only thing that makes good neighbours is tall fences. Good Luck