Xfce

Preventing the "Save Session" Checkbox from showing in the Shutdown Dialog

Firstly, create the kiosk directory with

mkdir /etc/xdg/xfce4/kiosk

Then create and edit the kioskrc with your text editor:

vi /etc/xdg/xfce4/kiosk/kioskrc

Place the following in your kioskrc:

[xfce4-session]
SaveSession=NONE

Also see this Askubuntu Article