There are two possible reasons why the application is started: It is saved in the last session or it is listed in the auto started applications. Follow 1 of the two steps below to get rid of the applications.
xfce4-autostart-editor and remove the application(s). You can also manually delete those files in ~/Desktop/Autostart and ~/.config/autostart.~/.cache/sessions/ directory when you're not logged in. And if you don't want xfce remember every session you should turn off (uncheck) “Automatically save session on logout” in Settings Manager → Sessions and Startup (tab General)There are two way to fix this: sudo and hal/dbus. Default starting from version 4.4 is hal.
You have to allow the user(s) to execute <prefix>/libexec/xfsm-shutdown-helper with sudo. <prefix>/libexec/ may be changed on your distribution, i.e. /usr/lib/xfce4/session/. Install sudo and run visudo (root) and add the following line (replace prefix with the correct path):
%users ALL = NOPASSWD:<prefix>/libexec/xfsm-shutdown-helper
Add the user to the users group (root):
gpasswd -a <username> users
Add the user to the power group:
gpasswd -a <username> power
When you logout and login again, the shutdown and restart buttons should be sensitive. For more information you can referrer to the xfce4-session and sudo documentation.
Make sure that the hal and dbus daemons are started on boot, and that you are running a recent version of dbus (at least 1.1). Refer to your distribution for exact steps.
In the steps below the groupname “power” is used. This is DEPENDING ON YOUR DISTRIBUTION.
Your /etc/dbus-1/system.d/hal.conf should contain a section similar to this:
<policy group="power"> <allow send_interface="org.freedesktop.Hal.Device.SystemPowerManagement"/> ... </policy>
Add the user to the power group (root):
gpasswd -a <username> power
When you logout and login again, the shutdown and restart buttons should be sensitive.
Enable the checkbox “Prompt on logout” in the session manager settings.