I've had this happen to me before and there isn't much one can do except be careful that your own dog doesn't get hurt. Speaking with the owners seldom helps , I find some people just don't care. One thing I did learn to do was carry a water pistol and I would use it when I had to, worked pretty well actually. I didn't like to try speaking sharply to the dog and try to chase it away because that seems to make our own dogs more alert to the fact that this should not be happening. A water pistol is quiet and doesn't hurt the other dog, but some just hate getting wet in the face.