I would have to agree with everyone else saying it would be far cheaper and easier to switch the power electronicly. Anyhow, you could use a spring loaded solenoid with the shaft glued well onto the 'on' side of the rocker. When power is applied, the solenoid pushes the switch down, when power is removed, it flips is back up because it is spring loaded. This is pretty expensive, though.
EDIT: It would probably be better if you just get a non-spring loaded solenoid and when you want the switch off, you reverse polarity of the solenoid, pulling the switch up.