Proxy server

Hi Group,

I have in my network a proxy server, but when users change the proxy
settings from their windows internet setting the can bypass my proxy, so how
can i force them only to go to the internet through my proxy.

P.S : I need some kind of configuration that i can apply on my switch or
router.

Best reagrds

-- 
Eng. Omar
Blogs and organic groups at http://www.ccie.net