well...change your shutdown and reboot rights? first check out where they are:
whereis shutdown reboot
and you get something like:
shutdown: /sbin/shutdown
reboot: /usr/bin/reboot
(and possibly some files ending with .gz but don't care about them now). then just, as root (change the paths if necessary):
chown root /sbin/shutdown /usr/bin/reboot
chmod 700 /sbin/shutdown /usr/bin/reboot
from now on, those files should be owned by root and their rights set so that root can read, write and run them and no-one else can...this is one way at least
