Big sticks work!! I do not walk my dogs with out at least a hickory cane or more likely a staff. My normal walking stick is about nose high on me and gives me enough reach to ward them off if they are just curious or lots of stick to work them over with if it comes to that. They are usually intent on my dogs and never see the stick coming. I have swatted their legs out from under them, driven the end of the stick into them, as well as just plan old hit them across the back or head. I have spent many hours working with my dogs to try to ensure they don’t cause problems and take what precautions I can to keep the irresponsible at bay.
The English have a history with the quarter staff might be time to knock the rust off of some of those old techniques.

P.S. No hope of putting the other owner down I guess? Since they are the problem not the dog that is the most logical answer.