I had 2 users on my system, one was and old administrator witch I didn't need any longer. So I created another admin, logged into it, forced the another admin to logout from terminal and removed it from the user list.
After that it still appearing on the "shut down" drop down menu.
For more info please see the screen shot attached to this report.
ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: nautilus 1:2.32.2.1-0ubuntu13
ProcVersionSignature: Ubuntu 2.6.38-8.42-generic 2.6.38.2
Uname: Linux 2.6.38-8-generic i686
NonfreeKernelModules: nvidia
Architecture: i386
Date: Thu May 26 13:02:07 2011
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release i386 (20101007)
ProcEnviron:
LANGUAGE=en_US:en
LANG=en_US.UTF-8
SHELL=/bin/bash
SourcePackage: nautilus
UpgradeStatus: Upgraded to natty on 2011-03-28 (58 days ago)
It has probably not been removed at all. Just restart the users and groups tool, and check that it's still present. Else, that's a bug in the users menu.