The roku box can only be turned off by removing the power cord from a power outlet. Or, if you have your roku connected to a uninterruptable power supply, you can probably look for a tool to turn off specific outlets on that UPS.
http://www.scribd.com/doc/38211573/UPS-PowermanagerII - that one has computer control for ON/OFF for the UPS system on page 36, so using remote desktop to your pc with the software installed will allow you to do that, provided the UPS is connected to both. Turning it back on might be a problem though...since you'd want the PC to be connected to the UPS too.
