That's a though one. There's no way known to me that will allow you to lock the computer without showing that screen. You could try setting the screensaver to 'blank' and at a time of 60 secs or so (with a password). That way the screen will go blank and the 'This computer is locked and in use' screen will be invisible. However, that will not prevent you nemisis from pressing the power-button on the pc.
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