Ignore this person. I bet you didn’t get this worked up about the 100 good comments you have received before this. These people exist and love being able to make these comments, but you can also just continue to run your business how need to, and focus on selling to customers who actually are going to purchase your product.
Does this fall under extortion laws? I remember an ebay buyer once threatened to give me negative feedback if I didn’t refund after they scammed me with a returned package that wasn’t the item I sent them. I filed a police report and used it with ebay and PayPal.
This might be slightly different. But I say tell them to sue you if you are sure they are scamming you.