Issue or symptom
You delete the previews in your library, or for a project, and disable "New projects automatically generate previews" in the Aperture preferences. The next time you open Aperture, the previews are recreated.
Solution
The preference New projects automatically generate previews only governs how new projects are handled. To prevent preview generation for projects that had previously had them, select the project in the Project Pane, click the Action menu (gear icon) in the upper right corner and disable Maintain Previews for Project. To prevent preview generation for all projects, highlight the Library in the Project Pane, click the gear icon and disable Maintain Previews for all Projects.
