Blocking MSN, YAHOO and other messenger clients

["Ashok Gautam\(gashok\)" <gashok@xxxxxxxxxxxx>]

Dear all,
Currently I am managing linux box having squid and other network with 
60+ nodes in a organization. But now the requirement came that to block 
MSN, YAHOO, GOOGLETALK, and other remaining messenger clients.

Please show me the way how to block all the messengers this might be 
already done by one of you friends.

Thanks in advance!!

regards,
Ashok Gautam