Restricting access by browser type

Hello, I'm new to the list so I hope this is not a widely known question.

Our internal users access the internet through a gateway server with
Squid installed. This have been working great for more than a year, I have
access restricted to some machines in our network. This is done not by squid's
ACL lists but by our Firewall-1 rules installed on the same system.

Recently some "smart" users discovered some windows proxy servers utilities
and are giving unathorized access to friends and such. I've been banging my
head finding a way to get around this problem. Maybe there's a way to tell
Squid to only service request from Netscape users since browsers send
the client/browser info/type with the requests. At least our http server logs
the type of client used in the request (Netscape/Explorer/winproxy/wingate/etc)

I'll really appreciate any help on this matter.

Juan

-- 
jleon@continental.fin.ec	Area Administracion Sistemas
				Sistemas y Proyectos
				Banco Continental S.A.