Mute!

At Sun, subscription to Sun internal/external aliases was managed using a centralized tool, which allowed you to enable, disable and select digests, control whether the alias was public or not.

Regularly email storms would erupt when a new employee or a clueless employee would email "subscribe" or "unsubscribe" to a large email alias (such as sun-all-employees). Many would respond to all with information on how to (un)subscribe. This would be followed by a storm about 'do not reply to all!" - flame - flame. The email thread would not die quietly.

Gmail added a nice feature recently, called "Mute":

If you're subscribed to a mailing list, you've no doubt been subjected to the 'thread that just won't die!' If you're part of a long message conversation that isn't relevant, you can 'mute' the conversation to keep all future additions out of your inbox.

So simple ... yet so useful.