Clip the power wire, or ground if needed, to the OBD2 port and feed it from the Ign wire instead of constant as it must be now, then it will shut off whenever the Ign is off, you can do this in several ways depending on exactly how you want it to work
You can also use a single throw, double pole, mini toggle switch placed where you prefer it and have it work either via the Ign, etc, or constant.