A hostile reply from a netblock operator seems like a perfectly valid reason to block their traffic.
That being said, a service like this doesn't come with any guarantees and if it'd disappear from the net tomorrow, I wouldn't blame the author. Blocking is a perfectly valid solution to this problem, but assuming malice isn't always the right answer.
Were I in this situation, I'd rate limit networks per /24 (maybe even /16?) as much as I could, and work together with antivirus companies to help identify infections of malware known to use the service to discourage criminals from abusing the system. I wouldn't even bother hosting the site on IPv6 since those addresses are supposed to be public anyway. The author clearly has more patience than I do.