Non-existent zone file

We run djbdns, and recently someone sent a huge spam campaign using a
domain we did not have a zone file for. We also did not register the
domain, but whoever did pointed it at our NS's and we were quickly
DDoS'd for a short amount of time until we created a dummy zone file
for said domain. Is there something we can do to help prevent this in
the future other than adding more NS's to handle the load or creating
zone files for domains we don't own or haven't registered?
Thanks!
-rich