dogs are cute, and I do exactly the same thing- if someone's walking with a dog, I'll always ask to stroke it. sometimes they'll say no (if the dog is feerce or in distress), but that's fine.

I don't say hello to people I don't want to either. I'll usually give like a nod or glance in their direction to let them know I've not completely ignored them and I am aware, or if I say something it will be what I'm doing.

for example I might say "I'm going home now", clearly indicating that I'm their for a purpose and that's what I intend to do. if they do insist on talking or asking questions, I just walk on