Proxy only for Netscape

1) I have configured my local proxy server using SQUID to deny access to
anything or everything.

2) I can very simply go to each users PC and set Netscape up to use proxy
server. Therefore that PC is controlled via the proxy server. So far so good.

3) However, if a smart user changes the netscape config such that he will
bypass the proxy server and use the direct connection, he now have full access
to the Internet again.

How do I fix this such that the user will not able to change the config on the
Netscape? Otherwise the use of the proxy would be pointless if the user can
change components on his PC.

Joel