I'm sure a lot of people won't like this, but hey, fight fire with fire. Send him an email - use formal language - stating how he's not entitled to a refund due

I'm sure a lot of people won't like this, but hey, fight fire with fire. Send him an email - use formal language - stating how he's not entitled to a refund due to the nature of your work, and making slanderous threats about damaging your reputation. Make sure to point out the legal implications of the threats he's making. Here's the fun part: end it with "P.S. I've CCed in my lawyer". Find a local lawyer firm and just make up anybodysname@lawyerfirm.com, paste that in the CC field. Worked for me once before. They get scared, look up the actual lawyers online - they're real - stop with their threats. This is cheaper than getting an actual lawyer but equally as effective at scaring them out of it.