IT Guys - How to block chat programs.

You may be able to log into your modem/router and access an allow/disallow list. This would result in the entire network loosing access to chat programs, so you would have to turn it on/off as other family members need it.

Also,
Even if you block the program itself from access to the net, some of these clients have web access which may get around it. For instance, you can log into AIM.com and chat from the browser. You would have to restrict it in the browser settings as well.

Not sure if that helps you at all, just thinking out loud.