Hmm. There should be an option in the BIOS for turning it off ... buy maybe there isn't.
I have a Gigabyte motherboard that does not have an option to shut off power to USB devices when it is "off".
I can turn off startup on a key press on USB devices but it starts up if I press a key by mistake on a keyboard in the PS/2 port ... *sigh*. Total bollocks.
Anyway, I also my PC and peripherals on a power strip with a master switch, which I use. (Of course only after a proper manual shutdown)