Most motherboard will leave USB ports powered on by default.
If there is an option like "EuP Ready" or just EuP in the BIOS (varies for different types of mobo) try enabling that and it should cut power to the USB ports on power-off.
It's an enrgy saving option wich is disabled by default, I think in ASUS motherboards it'c called something like "Energy Saver"
Hope this helps!