[squid-users] Transparency and blocking other proxies

[jimmy]

Greetings all. So we have transparency working for squid, but is there a way to
block the user from simply using a different proxy under preferences/etc... in
the browser settings. For example, if a user suspected that we were using a
transparent proxy to limit their access, they can have a different proxy setup
on a remote site and enter that address as the proxy address in their browser.
I know schools use squid, but how do they or anyone prevent this from
happening? Thank you.