Authoritative Server - Referrals to root

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We have problems with our authoritative only servers (BIND 9.3) sending
resolvers into loops.  If the root server delegates a domain to us, and
we do not host that zone, we reply with NXDOMAIN and hand back the root
servers list.  The root servers hand back our authoritative servers, we
hand back the root servers list.  Query rates for non-existant domains
on our servers are 100 times more than what any other queries on the
servers.  I think this is because the resolvers are just looping trying
to find answers.  What's the best method to resolve your authoritative
zones only and SERVFAIL everything else?  BIND 9.3.x