Put a small electric fence around the plants nearest the sidewalk and your front gate. It won"t really harm the dogs, but they won"t want to pee there ever again !! And try taking photos of the dogs and their owners, when these incidents occur-----if anyone asks, just say you"ve been advised to by your lawyer !! Neither of these actions need be too drastic, or hurtful, BUT it might set some dog-owners to worrying about the consequences of their actions !! If it was me, I would point a garden hose-pipe right at the dog-owner and say " Excuse me, but you are standing where I am about to irrigate ! ".