lucy24 - 3:21 am on Sep 7, 2011 (gmt 0)
All-purpose rule: Do not give Google a 404 if there is any alternative whatsoever. It will keep looking for the page forever. Or until the robot's warranty expires, whichever comes first.
If you can, tuck the relevant pages into separate, roboted-out directories. Any links to them should be no-followed. (Belt and suspenders principle. Belt and braces, for the Brits.) And then, to be safe, fire up the htaccess and block google from seeing the pages at all.
But first: if you do nothing, what does Google get? Are the pages set up so it's physically impossible to reach them if you're not logged in? If so, google should already be getting a 403.