If you have a soldering iron, the easiest way is to remove the green PTC fuse on OSPi v1.4. Once the fuse is removed, the on-board 5V will be disconnected from RPi. Here is an example picture of the PTC fuse:
To de-solder it, just apply soldering iron on one end of the fuse, and when it gets heated up, both ends will become loose, then you can remove it with a tweezer.