If you want to block Yahoo and MSN Messengers in a firewall use the following procedure this is very helpful in corporate environments.
Port to block for Yahoo and MSN Messengers
TCP: 80, 5000-5050
UDP: 1503, 3389, 5004-65535
Try to block the IP address range for msn messanger
Yes, you’re blocking a lot of IP’s but a large percentage of them belong to microsoft ( hotmail,msn,msn messager…etc )