Block chats

it can block chat / instant message clients, as they are not on important ports for your business.
use the firewall to disable all but ports required for email, web and ftp.
this will not stop web based chats, but stand alone clients will be blocked.

it may not even stop mozilla's chatzilla ( irc client ) netscape's icq/aol client, as the browsers may be able to route through port 80. ( web )

Also, Sonicwall provides filtering services for a fee. You can contact them or your reseller for subscription pricing and then you can not only block by port but you can also block by site on pre-determined and custom lists.

I've got one at my shop and I love it. You can almost hear the jackboots in the comms closet. LOL

For MSN, YAHOO etc you should block their ports.

For browser based chats
as http://www.chaosex.com
one should also block the site