I'd like to add some amplifying information :
Two companies are responsible for over 30% of the hits coming into Whois.
They are not major ISPs. This is almost three times as much as ALL AOL
users combined. What we are talking about here is abuse of a needed
service and as such should not be confused with peripheral business issues.
David H. Holtzman
Sr VP Engineering, Network Solutions
dholtz@internic.net
The solution to that problem would seem to be a rate-based filter. 
Since you're hitting a dbms, this shouldn't be tough to do either
I'd like to add some amplifying information :
Two companies are responsible for over 30% of the hits coming into Whois.
They are not major ISPs. This is almost three times as much as ALL AOL
users combined. What we are talking about here is abuse of a needed
service and as such should not be confused with peripheral business issues.
Any chance you'll tell us who they are so that we can block 'em, as they're
undoubtedly going to try to spam us?