So, using Sp@mX, I've dutifully reported them, and received quips from the domain admin of ISPsystem advising "we can't control this...". Please.
So, their provisioner is UUNET. With one SQL statement, I add the UUNET abuse info for this domain.
Bada bing, bada boom, now UUNET will also receive the abuse complaints for this unresponsive domain!