To Undo a Cookie
To undo the "Accept a Cookie" option while in Netscape, open the Options menu then Network Preferences. Go to Protocols and undo the "Accept a Cookie".

To Thwart All Cookies

1. Quit from Netscape Navigator.
   
2. In System:Preferences:Netscape, find the file "MagicCookie" and copy its name to the clipboard.
   
3. Drag the file "MagicCookie" into the trash.
   
4. Create a new folder in the Netscape Folder and name it "MagicCookie".

When you restart Netscape and it tries to accept a cookie by creating a file called "MagicCookie", it won't be allowed because a folder can't be replaced by a file of the same name. With this empty folder in place, you can even change the Netscape option back to its default of NOT showing an alert before accepting a cookie, because even if it tries, it can't do it.