You could also try disabling USB Storage (Also called Legacy USB) in the Integrated Peripherals page of the BIOS. You could also try disabling wake by keyboard and mouse in the power options page. Also disable all the C3/C4/C5 ect options in the CPU advanced options.
I know this can also have something to do with PSU's, as some keep mice powered up when the PC is off and some do not, due to the power always constant and (ON) for the +5VSB rail (+5V Standby). That is just how things are designed these days, some older motherboards have a jumper to enable/disable the +5VSB power.
If none of that works then your only solution would be a PCI/PCIE power USB controller card for the devices that bother you while the system is off and they are still powered on, they would be off for sure if connected to a controller card.