A perennial problem. Devices should detect when the PC goes into standby and shut off lights and stuff. For USB 2, they must reduce current consumption to a very low value to meet the spec, but most don't (I had an argument with a mouse manufacturer about this, but they didn't care about conforming to the standards).
Motherboards should also provide an option to turn off the power when in standby/off, like VirtualLarry mentioned, but apparently that's too much effort.
So, my usual solution is a bit of tape. Alternatively, if you don't want to disable it completely, a black marker pen can reduce its brightness.