I had something similar happen to me at THE BIG annual conference for my field. I was nervous about my speech, but I'd gotten through 3/4 of it when I presented a completely non-controversial slide. A woman in the crowd scoffed, raised her hand and told me that no one did what I had outlined in the slide and that I was wrong. I shrugged it off, said that was up to the individual and tried to move on. But the woman continued to argue with me. So I very calmly said, "That's your opinion and you're welcome to it. I'm not going to argue about what is a pretty innocuous topic, let's move on."
I finished the presentation with a smile on my face, but inside I was DYING. I hate public speaking anyway and it was pretty embarrassing to be contradicted like that in front of a room crowded with my peers. Several people approached me in the hallway and apologized profusely for the woman's behavior, saying, "That is just NOT DONE at this conference. Speakers are treated with respect." And apparently, several of those people approached the heckler afterwards, too, because she tracked me down the next morning and apologized for her actions.
I did not tell her it was "OK." In fact, I was still pretty irritated about it. But ultimately, I realized that this woman had made a jerk of herself, not of me. Same goes in your situation. This guy is a known as a mooching agitator. His actions don't reflect on you, they reflect on him.