Are you stating that the "host" pc has more than one NIC and you want to limit which one can share, or are you looking to limit which PCs can connect to the "host" pc and get online?
If it's the first one, then you can go to the options for the network adapter in question (right click on Network Neighborhood, go to Properties, then go to the properties page for the adapter in question and you should see a "Sharing" tab to enable/disable sharing) and you should be able to control which adapter is bound to the ICS function.
If it's the second, I would imagine that could be controlled by one of the following:
A. Removing the ICS function on the client
B. Examing the properties of the browser in the connections tab, and then removing any proxy/connection information
C. Adjust the network adapter properties of the client and remove the gateway information so they can't route out through the ICS box (if using a DHCP function from the hosting ICS box, then just use static IPs on the same subnet so that they can still communicate to each other on the LAN).
Bear in mind, I these are just *stabs* at what could be done, since I have only read bits about ICS but I am much more familiar with RRAS w/NAT (which ICS is a stripped version of) and other proxy/NAT/PAT systems.
EP, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS