This happened a few years ago, but silence was the most logic response to this person.
There was a woman who used to work at the same place I did, and she was a know it all, in a big way. I was in our lounge with 2 other people who were probably way more computer literate than I am, and her. Of course she places herself in our conversation, nothing unusual there, and proceeds to tell us there is more than one internet. I think I looked at her like she was nuts, and said, "What?, there is only one internet."
"No there is more than one internet." and she proceeds to rattle off a bunch of different providers, convinced they each have their own internet.
We got up and left, never saying another word, because we would have appeared more "brilliant" than she, had we continued.