In OS X 10.5, you can go to the System Preferences, and in there, go to the Network Pane. Select Airport and then the Advanced button. Within there you can deselect older preferred networks. I don't have any 10.4 machines laying around to see if it is the same there though, sorry.
yeh, u want to access it through the system preferences. if you try and adjust it from the menu bar you'll only have limited access to the network settings, something which has thankfully been remedied in Leopard. :-)