disabling caching on selected zones

[Ollie Wild]

Hi.  I'm running a simple bind configuration on my home network.  The 
bind daemon is authoritative for my local network.  Any requests outside 
my local network are forwarded to my ISP:

    zone  "." {
            type forward;
            forward only;
            forwarders {
                    xx.xx.xx.xx;
            };
    };

I also have a vpn to my office network, which I only enable 
occasionally.  The name of our office mail server resolves externally, 
but access is disabled.  Once the vpn is active, the same hostname 
resolves to an internal IP from which I can access mail.

The problem is that named caches the external IP, so everytime I enable 
the vpn, I have to restart named to access my mail.  Is there a way to 
explicitly configure bind to disable caching of results from a specific 
zone?

Thanks,
Ollie Wild