Citronella spray works.  I have a canister of citronella spray that I bought from my vet.  It clips on like a can of mace.  I was walking a dog this past week and another dog was off-leash and approaching us.  Another woman who was out walking her dog said that the dog approaching us had attacked her dog 3 times.  I did try tossing treats at the dog first to distract it but that didn't work.  He got about 5 yards away from us and I sprayed the citronella in his direction.  The first time he didn't stop but the 2nd time I sprayed it, he turned around.  Success!  It is not harmful to dogs but they dislike the smell.