Is there any way to block an individual website when someone is accessing it through a proxy server?

I already have HOSTS files setup but they don't work when the computer is going through the proxy server.
Thanks!- clara

The reason why a hosts file entry doesn't work when you're using a proxy server is because you're not really doing the DNS resolution locally when a proxy is configured. All you're doing is passing the URL to the proxy server and the proxy server does the lookup for you. So a hosts file is unfortunately worthless in this case. So what you'll need to do is either add the entry to the hosts file of the proxy server or use something like FoxyProxy addon in Firefox which specifically forces Firefox to perform the DNS lookups locally vs a SOCKS proxy.

Thanks so much FoxyProxy works!