[squid-users] Block all Web Proxies with squid.

Is it possible to block all Web proxies with squid ?
cose users can access bad sites and I can't do nothing !
I banned with url_regex for example the word "proxy"
and now users cannot access for examply this site:
http://www.proxytoplist.com/
but they can enter other web proxies like:
http://www.unblockmyspace.com/
or
http://anonymouse.org/

And I can't find and block with url_regex all web proxies is not possible
maybe there is another way ?
waiting for a reply
10x in advance

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