how to block web messenger services

[nawalmiftahi () gmail com]

Hi all , 
   i am a security admin,yahoo messenger and other chat services are blocked on our network, but we can access them 
thru web(ebuddy) , i wanted to know how can we make sure that these chat services (yahoo messenger, icq, msn messeger ) 
cannot be accessed through web as well , i mean how to block them.
thanks