Miran,
One option for powering down the SM200B is to shut down the PC controlling it. Most PCs allow you to select whether or not the USB is powered when it is shut down. If the USB 5V is turned off, the SM200B will turn off completely (I believe it is something like a milliwatt of power draw).
If a mechanical switch is what you want, any switch that is rated for at least 20V, 5A should be sufficient.
