I was able to block gtalk accessed through gmail ( following is how i did it )
In /etc/squid/special_url add the following .chatenabled.mail.google.com .talk.google.com
and in squid.conf add the following
acl special_url dstdomain "/etc/squid/special_url"
and deny acces to special_url
http_access deny special_url
However i am not able to to block gtalk chat client even after i blocked port 5222.