Disable reboot when ctrl-alt-del is pressed

systemd

ctrl-alt-del.target
    systemd starts this target whenever Control+Alt+Del is pressed on the
    console. Usually, this should be aliased (symlinked) to reboot.target.

gsettings list-recursively org.gnome.settings-daemon.plugins.media-keys org.gnome.settings-daemon.plugins.media-keys logout '<Control><Alt>Delete'

systemd

To prevent systemd reacting on on ctrl-alt-del key combination on a TTY, mask ctrl-alt-del.target unit (this in fact creates symlink to /dev/null):

systemctl mask ctrl-alt-del.target

Now check to ensure the symlink has been set/updated as expected:
 

ls -l /etc/systemd/system/ctrl-alt-del.target
lrwxrwxrwx 1 root root 9 Mar 23 14:51 /etc/systemd/system/ctrl-alt-del.target -> /dev/null

Ctrl-alt-del reboot has now been disabled. Test ctrl-alt-del to verify.

NOTE: systemd has a feature to still perform an action if ctrl-alt-delete key combination is pressed on a condition that user presses the keys more than 7 times in 2 seconds. See man systemd-system.conf(5):
 

CtrlAltDelBurstAction=
    Defines what action will be performed if user presses Ctrl-Alt-Delete
    more than 7 times in 2s. Can be set to "reboot-force", "poweroff-force",
    "reboot-immediate", "poweroff-immediate" or disabled with "none". Defaults
    to "reboot-force".

To prevent systemd from reacting on ctrl-alt-delete burst, do:

1. set 'CtrlAltDelBurstAction=none' in /etc/systemd/system.conf
2. reload systemd configuration with: systemctl daemon-reload

GNOME
 

To change this behavior to no longer reboot in GNOME on ctrl-alt-del, change the value for the logout key to something else. In the following example, it will be changed to ctrl-alt-shift-del requiring shift to be pressed as well.

Run the following logged into gnome for the user you wish to change:
 

gsettings set org.gnome.settings-daemon.plugins.media-keys logout '<Control><Alt><Shift>Delete'

Now test ctrl-alt-del.

This will need to be run for each existing or new user which you wish to change the ctrl-alt-del behavior for.
This is not ideal, as a disadvantage of this is that this can be overwritten by the user too..)

When it is desirable to disable the ctrl-alt-del behavior globally and for all users, follow the following procedure :

Create /etc/dconf/profile/user  file with the following content :
 

user-db:user
system-db:local

Create /etc/dconf/db/local.d/01-local  file with the following content :
 

[org/gnome/settings-daemon/plugins/media-keys]
logout='<Control><Alt><Shift>Delete'

Lock the logout property by creating /etc/dconf/db/local.d/locks/media-keys  file with the following content :
 

/org/gnome/settings-daemon/plugins/media-keys/logout

Next save and update the dconf database :
 

dconf update