godaddy spam / abuse suspensions?

or how about using an NS that returns ICMP errors instead of NXDOMAIN,
perhaps using anycast for reducing network load?

  ICMP is not particularly useful unless the nameserver uses
  connected sockets. Now that randomised ports are used this
  well may be true but there are still lots of nameservers that
  don't see the ICMP message even it makes it past the firewalls.