Blocking queries to certain domains

[Ali Eghtessadi]

Dear All,

I would like to know if there is a way to block or drop queries to
certain domains? I am trying to block my users from using Yahoo
messanger. One way is to block the IP address of the servers in the
firewall but because there are so many servers it is not a good solution
for us. I thought, may be using dns to block the entire domain would be
an easier solution.

It appears that all Yahoo's chat servers are part of the following
domains :

msg.yahoo.com
msg.edit.yahoo.com
messenger.yahoo.com

Your help is very much appreciated.

Regards,
Ali