The number one way to stop a thoughtful conversation is the insult your conversation partner, their choice in reading material, their friends, or their thoughts. Insults are a show-stopper. It only gets ugly after insults.
So step away from the temptation to tell that person that he is an ignoramus and politely ask about their sources.
Key word: politely.
Other useful words or phrases that will make you sound like you have a degree in counseling, are severely passive aggressive, or were schooled by Emily Post on the playground in first grade:
perhaps, have you considered, I wonder if, does this make sense, conversely.
And if you are worried about looking like a pansy for being polite, consider the alternative: a donkey's hiney. The last time I stared at a donkey's hiney, I gagged. The last time I stared at a pansy, nice thoughts rushed through my veins. You decide.