um if all of the puters are individually connected, then there is no way that one of them can firewall the others... it has no control of the flow of data. If I were you I would run to your local small puter store, and get the oldest piece of (>=486) refurb garbage you can find, and put 2 PCI NIC's of different brands in it. Then install OpenBSD and configure it something like this:
http://www.openlysecure.org/openbsd/..._firewall.html
Once you have this invisible bridge in place behind the router, you're able to specifically stop everything but that which you want going to the server, and make the client computers much, much more difficult to detect.
HTH
Ben.