This was always a huge problem with our old Collie, he got savaged by two dogs wandering round loose when he was a puppy (barely survived) and while he was never left loose and we managed to get him to the stage where he was OK with other dogs he knew, he'd always attack any strange dog that approached him and he'd be out to kill, not to tussle. I always warned people but they'd just fucking talk over me telling me how nice their dog was up until the point their poor dog got hurt. Ended up just getting out of bed to walk him at 2/3 am when there was nobody around.
Strangely enough, I don't have this fucking problem anymore now I'm walking either a doberman or a bulldog depending on which of the dogs I've got out. Shame really as they love meeting new dogs.