If you want no connection at all try this:
(been using Panther since WWDC, so I'm writing this up from memory)
Open System Preferences
Click on the Location drop-down menu and choose New
Give your new location a witty name like, "Workin' without a 'Net"
Click on the Show drop-down menu and choose Network Port Configurations
Un-check all network ports
Now whenever you want to disconnect from the 'Net, just go under the Apple Menu, choose Location then choose "Workin' without a 'Net"