One way to turn off touch screen on my Chromebook is to open the browser and go to chrome://flags/#ash-debug-shortcuts using the address bar. Then enable Debugging keyboard shortcuts and reboot your Chromebook. Once starting up, simultaneously press Search + Shift + T to deactivate the touchscreen. This shortcut key changes the touch functionality, so if you wish to reactivate the touch screen you only need to press the keys once more. This solution is applicable to almost all Chromebook models.
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