I definitely get the frustration. I tend to revel in the rare occasions people treat me rudely, though. It usually makes me smile and give a bit of a bewildered "are you serious?" look. I then will try to extend the conversation as long as I can to annoy the other person as much as possible. But I'm with you. I find this sort of thing pops up a lot with the "how are you doing?" pleasantries. If the other person won't even let me finish saying how I am and asking them how they are, I definitely change my tone with them and give a look that makes it clear I was still trying to speak. There's being nice and there's being a pushover. You don't need to be a pushover just because you stutter.