Log into your account at https://app.mangodisplay.com
Click on the display you want to remove.
You will now see the display layout screen. Click on the settings icon on the top right of the screen.
You will now see a new screen. Click 'Reset & Remove'
You will get a confirmation pop-up screen. Click 'Reset & Remove'
That display is now reset to its defaults (so it can be setup again if needed) and also removed from your display list in your account.