hmm a remaster would be needed i guess.
to begin you might make the "kioskuser" a separate group named "kioskuser" and allow that NOTHING eccept /home/"kioskuser" then as above make all directories not nesesarily read/write read only to kioskuser (as far as i know nothing outside /home), after that perhaps a chown -R root:root to several critical folders (such as /etc /usr/etc, /bin, /sbin, /usr/sbin...).
hmmm, might as well do the reverse chown -R root:root / then open permisions as needed, would take longer but be as safe as could be made.