Here's another approach - I just used this after a chat session with their domain registrar's support team: https://support.namecheap.com/index.php?/Tickets/Submit

Their identity is cloaked behind whoisguard, but it is not hidden from ICANN, I'm sure.

Fill out a complaint there - I included my most recent DMCA notice to the hosting company and a request the registrant be reported to whatever authorities need be, in hopes that they won't be able to register ANY domains any time soon.

We'll see if that has any effect. ;)