How to make ISA 2000 Proxy Mandatory

I’m working at a school that uses the ISA proxy server and has scripts that make the ISA server be what the web browsers go through.

The problem is that even with getting the IE7 and Firefox ADM packages installed to configure the browsers to use the proxy and force it, students can still download another browser and use that, such as Netscape, Flock, Opera, or use a registry hack to bypass proxy.

I need a way that makes all users go through the proxy and only the proxy, so I can continue to use the rules and blocked sites lists. If I can’t force proxy, I’m looking at getting a Barracuda unit from http://www.barracudanetworks.com/ns/?L=en . If anyone has any suggestions please do let me know. Thanks!