First off, I'm going to move this to the Generations board.
Secondly - You can remove them by going to each individual memory in your scrapbook and hitting the trash can icon. That permanently deletes the memory. Without a mod or until Seasons, this is the only way to delete them currently.
With the seasons patch coming in November, you will be able to turn off memories but until then if you are not opposed to mods and you're not doing a challenge file, there are several that offer the chance to remove and turn off memories including twallan's Master Controller.
