Ubuntu – Shutdown and User button missing in panel

Option 1:

Install gconf-editor, run gksu gconf-editor and on /apps/indicator-session/ supress the restart and shutdown.

I'm not sure about suspend and hibernate, maybe changing <allow_active> to no on /usr/share/polkit-1/actions/org.freedesktop.upower.policy will do the trick.

Option 2:

Remove the Indicator session package and add desktop shortcuts to logout and locking the screen.

The command to logout is gnome-session-save --logout-dialog and to lock the screen use gnome-screensaver-command --lock.

In case integration to the indicator menu is not a prerequisite we can always define a custom launcher or a custom application indicator calling the follwing command to allow system restart only:

/usr/lib/indicator-session/gtk-logout-helper -r

See also: What is the command to open the shutdown dialog?