Preventing NetBios requests dialling out

I have a W2K server that has an ISDN modem attatched to it. Whenever a client sends a NetBios request on Port 136, 137 or 138 to the server it automatically passes that request to the ISDN modem and it initiates a cial-up to the Internet when this request had absolutely nothing to do with an external address. Is there a way that I can force WIN2K server to not forward these Netbios requests to the modem and only allow requests on legit ports such as 80 etc. I am also using ISA

Many THanks

Isaac de Abreu
Johannesburg – South Africa