Press the power button on the screen. If you press and hold this for three seconds, you will get a menu where you can choose between black screen or lock screen.
Option with the remote control.
Using the "Button 11" on the remote control, you can choose to black out or freeze the screen. Keep five fingers on the screen and click the cogwheel on the left or press the cogwheel on the remote control. Under 'General' in the section 'Remote control' you can select the function of this button