Chromebooks do not offer a built-in way to permanently turn off the touchscreen on chromebook by default settings. You may disable it temporarily by turning on the Debugging keyboard shortcuts from chrome://flags/#ash-debug-shortcuts, and then restarting your Chromebook. When the machine starts up again, hit Search + Shift + T to turn off the touchscreen. The touchscreen remains disabled during the session, but it might be re-enabled automatically after updating, restarting,g or changing flags. If you want to keep it turned off, you will have to resort to the shortcut every time the touchscreen is switched on.