To change the reboot behavior to no longer reboot in a tty on ctrl-alt-del, point the symlink to /dev/null instead and the key combination will no longer reboot the system:

ln -sf /dev/null /usr/lib/systemd/system/ctrl-alt-del.target

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

SUSE:/home/SUSE/ #  ls -lah /usr/lib/systemd/system/ctrl-alt-del.target
lrwxrwxrwx 1 root root 9 Jul 11 09:23 /usr/lib/systemd/system/ctrl-alt-del.target -> /dev/null
You have new mail in /var/spool/mail/logan

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

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