I don’t want to rock the boat, but is it blackmail? Is the theme perfect in every possible way? Is it possible that the guy isn’t trying to get a free theme, but that he’s actually having a lot of problems with it, which is why he wants a refund?
His tone is obviously not very nice in the slightest, but imagine this…
You’re in McDonalds and your burger is raw and there’s a bunch of hair in it. You ask for a refund and they decline. You then tell them that if you don’t get a refund, you’re going to leave bad reviews everywhere and expose the issue to the public.
Is that blackmail? Not really. I’m sure you’ve had plenty of happy buyers with no problems, but is it completely outside the realms of possibility that the customer is actually having problems with the theme, even if it’s just a case of ‘user error’?
That’s a bit different then! I would tell him that any act of retribution will be avenged a thousand fold, and that he will beg for release from the torment and the anguish that will haunt his nightmares… from this day forth and for the foreseeable future.