How do you selectively block Facebook chat from a network while allowing other Facebook access?

As of October 2009, it appears that Facebook is using the following range of IP addresses for chat. I am not certain if they are using these IP addresses for any other features, and I am not certain if they have any alternate ranges.

69.63.176.161 - 69.63.176.199
(channel01.01.05.sf2p.facebook.com - channel39.01.05.sf2p.facebook.com)

If you block these IPs at a firewall, chat should be disabled.