Proxy service used as Internet proxy server

System

TS 5.5.2 Win2K
We noticed in our setup (which I think will happen in any other setup also) that if I use the IW hostname as Proxy server in my browser settings with port 80, its able to function as a normal proxy server. i.e. anyone can specify this server to be the proxy and then can access Internet.
This is not desirable in our setup.
Is it possible to supress this unwanted feature?

TIA