Your PSU should have a switch on the back of it. When you power down the PC turn the swith to the off position and see if that stops the fans. Also, you might want to plug the fans into your power supply via a molex connector (using adapter) and that way it will shut the fans off when the power is turn off. It looks like this APEVIA Model CVT43 Power 4 PIN Adapter TO 3 PIN Fan Adapter they sell for like $3.99.