Does anyone knows which port that messenger used?like msn or yahoo messanger. And which property that i should set the firewall filter rule to block someone cannot chatting using msn or yahoo messanger? sorry,i’m newbie in mikrotik n networking. Thanx for ur help… 
Here is a list of Ports used and common applications including chat and messenger programs.
http://www.practicallynetworked.com/sharing/app_port_list.htm
Example: (Yahoo Messenger Chat)
/ip firewall filter add chain=forward dst-port=5000-5001 protocol=tcp action=drop