How to forget a wireless network?

Is there a way to "forget" a recently joined network so that your computer will not automatically log-on to it? Once when our network was down I joined an open wireless network so I would have some internet access, but all our administrative stuff is IP restricted to be accessed only from our password protected network.

I like the feature that remembers previously joined networks and automatically logs me on, so I don't want to disable that feature completely (as described here: http://discussions.apple.com/thread.jspa?messageID=2443162&#2443162 ) and I have it set to ask me before joining an open network, but since I have already agreed to join this one undesirable network once, I can't find a way to keep my laptop from joining it automatically.

Thanks for any replies.

-Lewis