I feel your pain. The tough part is realising how pointless those "someone is wrong on the internet"[1] discussions are. I get embarrassed looking back at situations like that, and think "why did I even care if this random person didn't get what I was saying about <thing-we-were-arguing-about>?"
Oh, I agree. I was as dumb as him for participating in it. If I'd any sense, I'd never have engaged with him after it became clear that he was fixated on something completely separate from the actual point.
[1] http://xkcd.com/386/