You can remove ubuntu-desktop
and simultaneously auto-remove all its orphaned dependants:
sudo apt-get autoremove ubuntu-desktop
Add the --purge
option if you also want to remove the configuration of the affected packages (and not keep it for possible later reinstallation).
If you have any other Desktop remove them as well. if you reboot after this you should have no GUI to log into. If you purge a program rather than just removing it you also remove any config files that may remain.
As Rinzwind suggests try sudo apt-get remove gnome-*
.
Before Ubuntu 16.04 it also provides a special kernel package for server installations, linux-image-server
:
sudo apt-get install linux-image-server
and reboot.
Then I suggest you install the server applications you want, like ssh-server
.
But as always it's better to make a clean install. It gives less risk of errors and broken packages.