I stare down the person until he/she starts behaving properly. Then I listen.
A man builds. A parasite asks 'Where is my share?'
A man creates. A parasite says, 'What will the neighbors think?'
A man invents. A parasite says, 'Watch out, or you might tread on the toes of God... '
It's variable, but generally I shut down entirely and refuse to let myself react until they cool down and start speaking rationally. It usually gets them to stop pretty quickly if I don't give them any emotional fuel (or irritates them further, at least briefly, which is amusing).
Rarely, I'll cry, but that takes a lot and pretty much guarantees I won't speak to them again.
...Lets just say... Not well... My mental reaction is often violent, but my course of action is most often to tense up then just stand there glaring at the idiot.
That used to be my reaction. However, I then went to work in sales with a very aggressive 8w7 manager who yelled at me at the top of his lungs everyday. After a while, I got pretty damn used to yelling.