Judy:
Locking down the computer can be fairly easy. If you use Zone Alarm Pro (paid edition) you have full control over where the computer goes or does not go. There is a section in there that you type either where it can go or can't go. In your case once you decide where it can go it would be easier to enter that information. Other software firewalls also would probably have something similar. I'm not sure so won't comment though. Also there is opensource software that can be used to achieve the same thing. Let me see if I can find it and I'll let you know. By the way -- if you are using a fixed IP on the computers AND you have access to the router you can do it that way on an allow basis.

Garry