I've looked into this myself.  There are some clues here:
http://www.gnome.org/start/2.0/menuediting.html

On the one hand I hope that Gnome 2.10 will allow menu editing again.
Gnome 2.4 had it but it went away when they started implementing open
desktop standards (to,among other things, make Gnome and KDE more
compatible.)  On the other hand I'm curious so I'll look into it this
weekend and let you know if I find anything out.

Brady

On Fri, 2005-02-04 at 13:18 -0600, Dan Drake wrote:
> Here in the math department they use Fedora Core 3, running Gnome 2.8.
> In my Actions menu, I'd like to remove the 'Lock screen' option, because
> (1) I never use it and (2) the 'Log out' option is just below it, and I
> sometimes accidentally lock the screen when trying to log out -- then I
> have to type my password to unlock the screen and *then* log out.
> 
> There seems to be no way whatsoever to change this menu, which I find
> extremely irritating. It's *my* session. I should be able to configure it
> the way that *I* like it.
> 
> Am I missing something? This seems like it should be super easy (like
> power-cycling my cable modem, hmmm...).
> 
> Thanks,
> 
> Dan
> 
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-- 
Brady Hegberg <bradyh at bitstream.net>