Regardless of how obnoxious the customer was at the convention, you took the money. Therefore you were under an obligation, no matter how mindbogglingly annoying the customer is, to complete the work you were paid for.

If he was grating your nerves that much, then refund him and take away a valuable lesson. Or better yet, don't take the commission in the first place. A business in art sucks, people get irritating customers all the time. I do, I've gotten customers I've wanted to slap in the face with my keyboard. But that's the catch with art. And you have two options. A, put on a smile, do the work, and get them out of your hair. B, refund them and get them out of your hair.

But I find this whole addition VERY unprofessional. You're playing the blame game and trying to make the commissioner seem more horrible than you and frankly, it's childish. I don't like the idea that if I complain about an artist COMPLETELY IGNORING ME that the artist will turn around, dig up dirt, and go "BUT YOU DID THIIIIIISSSS THEY'RE THE BAD GUY."

If they were such a difficult customer, make an entry. But you just look like you're trying to pass off responsibility.