Set their ie to use a proxy server and it will appear that the internet does
not work, but email does. The net still works, just that IE tries to connect
to a bogus proxy server and gives the page cannot be displayed.
In IE: tools, internet options, connections tab, lan settings, check the use
a proxy server box, put in 127.0.0.1 for the ip address and 1111 for the
port, ok out. Now IE will not connect to any websites. You can then use a
program like spybot to lockout all users from changing that setting.