how to allow only some users to access yahoo messenger

["amit ash" <ashkadabba@xxxxxxxxxxxxxx>]

Hi members,

I have squid 2.5 running on suse linux 10.0. my users login into 
their workstations through active directory on windows 2003 
server. Is there anyway i can allow or block users from accessing 
yahoo messenger specifically based on the username with which they 
have logged-in in their workstations.

Amit Ash