Why do you want to start a complete KDE session if the only thing that should be accessible is your program? Wouldn't it be better to just use a plain X session and run only your program (and maybe a lightweight window manager) on it?
I tried some other windowmanagers, too. But then i have to find a solution for nearly the same problem just for another windowmanager. Because I found the most relevant informations for KDE, I chose the KDE-solution. Unfortunately I just found most of the solutions for KDE, but not all. ;-)
Then configure fvwm2 in a way that the user cannot start anything else (I hope that is possible).
This is the problem I meant. I also don't know jow to configure this for fvwm2 or any other lightweight windowmanager. If someone wants to give me a hint to increase my knowledge of some other windowmanager than KDE I would be happy of an alternative solution.
Do you really need a window manager at all? You could also start your application so that it fills the whole screen (using the -geometry X option). If your application exits, the session will terminate and you get back to the login screen. Mario