Turn down the brightness and contrast on a monitor it the lowest visible settings.
It is amazing, but most people will not try to adjust it. If they do adjust the settings back, keep adjusting the monitor every time they shut their computer down.
Usually, the person will start to wonder why their monitor will not hold its settings instead of just checking the adjustment.
Many new monitors with digital controls can be adjusted even though the computer is off. Turn on the monitor, open up the menu, adjust the contrast, turn it off.