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.
It feels like this sort of data (even if only providing order of magnitude estimates) would help greatly with deciding on appropriate rate limits for small operators who don't have the time to research all the traffic they're receiving.