You can hold down the shift-key while lauching Aperture - this will defer the generation of previews. Then check your Aperture Preference Settings again, if you really disabled the generation of previews.
If you have a protection problem with the user preferences file, Aperture may not have been able to save your settings. In this case it should help to remove the preferences file "com.apple.Aperture.plist", see the paragraph "Delete the User Preferences" in :
Aperture 3: Troubleshooting Basics: http://support.apple.com/kb/HT3805
This will reset the Perferences to the default values, so you will have to set the Preferece Settings again. Make sure you launch Aperture with the shift-key hold down, until the preference settings are fixed.
If you really need tp have previews, will depend on wheter you have a rereferenced or a managed library. If your library is referenced, I strongly recommend to have previews, like Allen stated. With managed library previews are optional, but you will not be able to use your images in the Media Browser.
The Preference New Projects Automatically Generate Previews only affects new projects (as the prefereence says)
To stop current projects from generating previews you need to go to the LIbray tab of the Inspector and unset
If you select the Projects icon and unset this all projects will stop maintainnig previews. You can also have just individual projects maintain previews by selecting the project and
Previews are not strictly necessary in Aperture. However, without them, there are some things you can't do, including:
* Sync photos to an iPhone or iPad via iTunes
* Drag a photo from Aperture straight to the desktop
* Search for photos in Aperture via iLife, iWork, or Finder
So, obviously, preview are recommended. But again it is possible to operate without them... I did so for a long time.
FWIW my library of 40,000 images has about 60 GB worth of previews. Most of my shots are with a 18+ MP cameras and the previews are quality 8, give or take. That's not much disk space these days.