Usually, people are not rude to me because I take pains to be polite to everyone. But in the rare circumstance when it happens, I smile and wish them a good day. Or, whatever pleasantry might be appropriate in the circumstance. Often, they are startled by my polite response. I suspect people who act like a male sex organ, which has a head but no brain, are looking for an opportunity to blow off their political steam. Your polite response removes that as a possibility and they don’t know how to react.
I’ve watched my friend utilize this technique with much success. People expect rudeness in response, and when they get the opposite, they often become dumbfounded just as you’ve pointed out.
“...I smile and wish them a good day.”
A sad smile and “I’ll pray for you” is my favorite gouge-back.
When someone pi$$e$ on my leg, I pi$$ in their face.
But then again, I am a proud member of the undesirables, thank you Hilary Clintoon. (Spelling intentional.)