How do I make a bigger dog back off without hurting anyone?

I know you all understand the struggle of walking small dogs when some jerk lets their big one off-leash. I don't usually have a problem with this in a public park, thankfully, but it's happened several times while walking around my subdivision. Of course, the owners never come to retrieve their dog or give me a hand. So far, the dogs haven't been aggressive, but mine are always freaked out. I have two Chihuahuas and two hands, so I can only pick up one for safety (usually the one who's most scared).

What can I do to actually scare away or make these dogs back off? I don't want to hurt them when they aren't trying to hurt us, but I know it can only take one second to start a fight. I've tried yelling and waving my leg at them like a lunatic. Today I actually pressed my show into a dog's face and he just kept going for my girl. I threw a bag of poop at him, and that at least distracted him. So tell me what I need, please. I just can't bear the thought of physically harming a dog until they give me a reason, and I don't want to end up with the owner calling the cops.