I've had a bunch of run-ins with dogs on the trail, and the only thing I can tell you is that you will probably never change anyone's behavior by yelling at them, so if you're going to do that, just admit it's to blow off your own steam. People are going to let there dogs off a leash in a park. The dog wants to run and if it isn't properly trained, it's going to cause trouble for someone eventually. The best I can think to do anymore is just let it go unless they say something to me first, and then I tell them without any emotion at all that they are irresponsible and stupid people risking their own pet's life because they're too f*cking cheap or ignorant to get him properly trained. It's amazing how well you can catch people off guard with a comment like that if you say it in a calm, monotone voice. I'm not going to fight some d-bag over something like this, but to be honest I don't give a f*ck about how some jackoff like that perceives mountainbikers either. I'm not a representative of a "riding community" or anything like that and I don't want to be -- I just want to be left alone to ride my bike on trails where it's perfectly legal to do so. Nine times out of ten, the owner is cool about it and apologizes, and I always tell them that it's cool because the dog is just having fun. And if they don't say anything, neither do I. It's only when they feel like they have to say something to me. And then I don't care about changing their opinion or even giving them something to think about. I just want to tell them they're an a$$hole and move on.
And another one of my favorites -- I occasionally poach illegal trails (we all do) and I've occasionally had run-ins like these on those trails. A little while ago, this lady started yelling at me when I came around her and spooked her dog. She was yelling about how this trail is off-limits to bikers or something. So I said, "Yeah, that's true. But it's also illegal to have your dog off a leash, so why don't we both just shut the f*ck up and move on?" Then I smiled at her and gave her the thumbs up and took off.