Change the following line in file /etc/inittab:
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
Comment it out with # to disable Ctrl+Alt+Del altogether or replace the /sbin/shutdown -t1 -a -r now part with a command of your choice. This way you can first unmount NAS shares before rebooting forcefully or use the halt command instead of -t1 -a -r now.