It can not be powered down because the aquaeo will always get power through USB. This allows the device to use timer actions or the remote control to start the PC.
In the information pages tab you could upload a black image as standby screen and as Dundys said in the user interface tab you can control the backlight settings for the standby mode. You can make the device look like it is completely off but as it will not be off
To achieve this you would have to remove the red wire (+5V) from the USB connection. If you do so, isolate the ending of the wire to avoid short circuits!