How to block DNS record scans ?

[Sylvan Andrew]

 Hello,

  Is their  anyone who could help us it would be much appreciated. Two of 
our DNS servers are continually getting scanned with some type of script 
that trys every combination possible from A-Z.
 Rather than limit the amount of DNS requests our servers handle on a time 
basis is there anyone who knows a way to modify the response to a entry 
record not being found ?
 Basically we'd want it so that if it was a valid entry bind would reply 
straight away, if it was a invalid entry we'd like rather than a immediate 
'not found' response to modify it so it just times out.
 Does anyone have and ideas where in Bind we could modify it to do this ? 
Does anyone have any other ideas to combat this problem ?

Thanks for your time.

Regards

Sylvan