how to identify DNS request source?

[Hostmaster]

My named-auth log shows repetitive requests (every 5 minutes) from an
IP address for both a forward and inverse record. We are not approving
the request and it is merely loading our name server and growing our log
file. I am unable to determine the source of this request. nslookup yields 
no information. Using Arin I have been able to find out who owns the IP 
address block which includes the requestor's IP address. How can we 
find out who and what the offending culprit is? BTW we are not
running dynamic or cahced name servers. Any advice would be most 
appreciated.