[root@localhost ~]# systemctl mask ctrl-alt-del.target
[root@localhost ~]# ln -sf /dev/null /etc/systemd/system/ctrl-alt-del.target
Now you can try pressing "Ctrl + Alt + Del" key combination and you will see instead of rebooting system it will log below messages to log file "/var/log/messages":
May 18 13:16:07 localhost systemd: Received SIGINT.
May 18 13:16:07 localhost systemd: Failed to enqueue ctrl-alt-del.target job: Unit is masked.