Here's a failsafe plan for the next time he interrupts you. Just turn to him and say: "I can't talk and play at the same time." You may need to practice it quietly on the gig for a few days beforehand, because it's very difficult to say while you're playing. You can also silently rehearse it away from the piano, your hands and feet moving as if jamming, your lips moving as if talking. This will work just about anywhere in the library, on a park bench, or even in bed before you drift off to sleep. Try wearing your gig clothes to make it more realistic.
If you can't pull it off when the time comes, it may be easier for you to sing the line, a slight blues inflection adding to the emotional impact. Either way, you'll need to follow it up with a long medley to be sure there are no gaps in your playing; no threat of further conversation. I'd suggest playing a little Sun Ra, some atonal free improv, and a few particularly inaccessible originals of your own. Your antagonist should lose any interest in hanging around, and you'll once again be safe from direct human interaction.
I'm curious: If annoying people like him were to stop asking you such ignorant questions, do you think you might feel like playing an extra hour or two just for fun?
Have a question for Mr. P.C.? Ask him.