edit the following line in /etc/inittab
id:5:initdefault: and
change it to
id:3:initdefault:
so if you reboot the default running level is 3 which is not GUI
For Debian 8-9/Jessie-Stretch use the following command:
In order to make text boot the default under systemd (regardless of which distro, really):
systemctl set-default multi-user.target
To change back to booting to the GUI,
systemctl set-default graphical.target