Drastic as it sounds, I would just cut the +5V trace on the board. There is absolutely no reason for the sanguino to be powering to usb line.
Cutting the trace takes 2 seconds with a pocket knife and costs nothing. Once cut, never have to worry about frying a usb port, blowing the 7805 or forgetting the adapter.
Cutting the trace takes 2 seconds with a pocket knife and costs nothing. Once cut, never have to worry about frying a usb port, blowing the 7805 or forgetting the adapter.