To turn off the touchscreen on a Chromebook, the most common and straightforward method is to enable a Chrome flag for debugging keyboard shortcuts and then use a keyboard shortcut to toggle the touchscreen on and off. Here are the detailed steps:

1. Open the Chrome browser and navigate to "chrome://flags".
2. In the search box, type "Debugging keyboard shortcuts".
3. Enable the "Debugging keyboard shortcuts" flag from the dropdown menu.
4. Click the "Restart" button to reboot the Chromebook.
5. After the restart, press the keyboard combination Search + Shift + T to toggle the touchscreen off.
6. Press the same combination again to turn the touchscreen back on.

Note: The Search key may look like a magnifying glass or a white dot depending on the Chromebook model. This method works on most Chromebooks and is reversible anytime with the same shortcut.

Some Chromebooks may also have native touchscreen toggle options in the Settings menu under Accessibility or Device sections, but this varies by model.