I use squid in my network for sharing internet, i use acl's to block websites like porn,social networking etc..
When i was monitroing squid traffice i came to  know that one of the client has gtalk app installed and it was working fine. I was wondered how could this happed, thing is i blocked gmail. but gtalk apps works connecting to gtalk server i.e talk.google.com talkx.l.google.com. Then i simply wrote acl to block these servers. That's worked like charm. Try this.

[root@ftp]# vim /etc/squid/squid.conf

acl gtalk dstdomain gtalk.google.com talkx.l.google.com
http_access deny gtalk

Now try signing gtalk, it won't .... enjoy.

gil...