Just had the same problem on another PC and solution was in the BIOS "power section" enable ErP/EuP this turns the power off to the USB ports in shutdown. Check your BIOS to see if it supports this.

"ErP/EuP, stands for Energy Using Product, was a provision regulated by European Union to define the power consumption for the completed system. With the great quantity and the popularly using of the electrical products nowadays and the continuously increasing of the electrical products categories in the following 20 to 30 years, European Union decided to establish an effective strategy to solve the energy consumption problem. That is, according to ErP/EuP, the total AC power of the completed system shall be under 1.00W in off mode condition."