disable changing background wallpaper

Hi list,

How can I prevent that a user changes the wallpaper by using a dcop call (dcop 
kdesktop KBackgroundIface setWallpaper foo.png). You can do that quite simple 
using kolourpaint or konqueror (right mouse -> action -> set as wallpaper).
After restarting the profile I see the old wallpaper again, because kdesktoprc 
is immutable.
However, during a session everyone can change the wallpaper.

Any ideas?

Thank you :)

Michael Hartmann