I'm not aware of any ready made solution but it's only bad design that allows it to happen anyway. A PC USB port should not allow power to flow back in to it.
If you use the diode trick, it is very simple to install, just open the keyboard, find the wire carrying 5V at the end of the cable, cut it and fit the diode in-line across the break. There are only 4 wires in a USB cable and one will be connected directly to the PC case. If you have a testmeter, measure the resistance from each wire to the case and the one with the lowest reading (should be less than about 3 Ohms) is the 0V wire. Then measure the voltage from the 0V wire to the other 3 wires, the one with 5V on it is the one to cut. The diode costs pennies but you must use a Schottky type, a normal silicon diode will drop more voltage, typically about 0.6V which might leave the remaining voltage a bit too low for some USB devices to work.
Brian.