Jessie/stretch is built on the systemd style boot and there is no more /etc/inittab. The behavior is controlled by /lib/systemd/system/ctrl-alt-del.target (not /etc/systemd/system/*): Type
ls -la /lib/systemd/system/ctrl*
lrwxrwxrwx 1 root root 13 Aug 30 23:04 /lib/systemd/system/ctrl-alt-del.target -> reboot.target
To change this we need to remove the current link and replace:
> sudo rm /lib/systemd/system/ctrl-alt-del.target
> sudo ln -s /dev/null /lib/systemd/system/ctrl-alt-del.target
> sudo systemctl daemon-reload
...so to disable Ctrl-Alt-Del we point the target to /dev/null i.e. to be completely ignored and again the last command 're-reads' the configuration and applies the change.