Forcing an user to only one window manager

[pedro noticioso <cucnews yahoo com>]

I am in need to limiting an user to only one window
manager and came up with this little script to
hopefully add to all files in Sessions directory

#!/bin/sh
   if [ "$USER" = "john" ]; then
       exec /etc/X11/xdm/Xsession qvwm
   ELSE
       exec /etc/X11/xdm/Xsession IceWM
   fi

   So all users may be able to choose whatever they
want but john is forced onto qvwm

but the result is not what I hoped for, may someone
please help me with this?

thanks!

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