How to Block all websites except one website in a LAN of Ubuntu 11.10 Clients ?
One of the simplest ways is to set up a squid proxy, and put rules in place to allow only the site you want. Then block access to all ports on your firewall for LAN PCs (you may need to consider if your clients need to access any other internet services). Finally, push out proxy info using wpad.dat or dhcp option 252, both of which should work for linux I believe.
A bit more info about your scenario would help tailor a better solution.