Given the situation, perhaps you could set up a resolver that blocks, or that's behind a packet filter that blocks, the IPs of the name servers they're using. That would catch it at the NS lookup, and would be no extra traffic, unlike whois.
- Milter to block registrars James B. Byrne
- Re: Milter to block registrars Bennett Todd
- Re: Milter to block registrars James B. Byrne
- Re: Milter to block registrars Eliezer Croitoru
- Re: Milter to block registrars Wietse Venema
- Re: Milter to block registrars LuKreme
- Re: Milter to block registrars Robert Schetterer
- RE: Milter to block registrars Marius Gologan
- RE: Milter to block registrars James B. Byrne
- RE: Milter to block registrars Bennett Todd
- RE: Milter to block registrars Marius Gologan
- Re: Milter to block registrars Patrick Ben Koetter