I am wondering how a water squirter might go. Dogs often hate having water squirted in their faces. What do people think of this?
Also, how about one of those cans of pressurised air. I use them on my computer keyboard and they have a long, thin nozzle. I know a few times I've almost jumped out of my skin when my son has directed a quick shot of spray at me when I wasn't expecting it. The problem with this of course would be that the other animal would need to be close, but a horn or a water pistol you can use at reasonable distance.
And of course, everyone knows that if dogs DO start a serious fight not to try and step in between them. Hosing is about the best but obviously on a walk that's hard to do. Sometimes dogs will have a skirmish that is scary but they'll release. If owners get in a tizz (understandable) then this can make it worse. One of the occasions when a spray WOULD be very useful but then you're likely to get your own dog as well of course.
I honestly wish people had to apply to have dogs these days. Really committed pet owners would mind I believe.