I just don't show any reaction. My face is a blank slate.
I consider this part of my skills
Sometimes, I get a pt who's tossing out insults and stupid comments and you can see from the expression on their face that they are enjoying what they are doing... they may be smiling or looking at me sideways with a gleam in their eye. You know the type I mean. You want to smack 'em! But, my brain is trained to shut off my facial expression when they do that. I have it mastered!
Sometimes, I even call their bluff, "Joe, I do believe you are trying to get a reaction out of me." I say this very matter-of-fact. This ruins their fun. If I know their game, they know they won't get what they want from me.
I had two sisters come to visit their mom. Both were totally obnoxious, but one more than the other. The more obnoxious one kept spitting out the most ignorant comments, obviously trying to wank me off (she darn near did it, too!). She eventually chilled out because I totally ignored her: I directed all my questions to her sister, I would only look at her sister and in no way did I acknowledge anything she said. I even turned by body so that I "edged" her out.
I've said a hundred times, "don't feed the beast".