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Oh, how welcome this is. Microsoft Research have a free NoReplyAll Outlook add-in download available which, as you might have worked out from the name, stops people replying to all the recipients of a message or forwarding it on.
Get the add-in and some more info here from the Microsoft Research website.
Haha, this will now be installed on anyone's machine that does this. If nothing else but to put them into a security group named 'ReplyAllAbuser'.