Writers: How do you deal with accusations of, "You stole my idea!"

Today I've been dealing with a guy who feels I used his idea for a recently published story of mine. (Just to be clear: I didn't.) Anyhow, it seems that this guy has a similar setup for a novel he's been working on, and someone who read his novel and heard about my story wrote to him to say it looked suspicious, and the third-party guy thought I had this similar-story-guy in my summer SF Writing Workshop (I didn't - he was in Kij's Novel Writing Workshop, so I never saw the book, outline, or any of that).

So I wrote to the similar-story-guy to clear things up, and now it appears that he thinks I'm a liar and a thief.

Egad, Charlie Brown.

He went from accusatory and "shocked" at my taking his idea to passive-aggressive a-hole during the course of the conversation. I feel I could have handled this better, but at least I did delete such phrases as, "your Machiavellian little mind" before sending the messages. Ahem.

As I publish more and teach more writers, I expect this kind of situation will come up more frequently. I imagine that John Scalzi hears from half a dozen writers every day with similar accusations.

Writers: Have you had to deal with such situations? If so, how did you handle it? I'd like to be the paragon of gentlemanly and instructive without telling the accusor to piss off.

Thanks,
Chris