I'm a little late getting back to this thread, but both of you are kinda missing the point.
What I am doing is installing several wall jacks in different parts of a building, with each leading back to a common router sharing an internet connection. People may plug computers into these jacks and access the internet, or even attach routers to them and create another network within the larger one, but I do not want anything that is connected to one jack to be able to interfere with anything that is connected to another. Basically, I do not want anything connected to one jack to pose a security risk to another. My main concern would be someone connecting to one jack and someone connected to another jack attempting to do malicious things to their computer.
I assume the easiest way to do this would be to use the router to block everything expect for HTTP, however this would limit the computer from doing ANYTHING except for standard internet access. I would still like them to be able to upload files to a sever, IM, etc. I only don't want them interfering with each other.