The best OBD safety mod is something unique that you do yourself, no one will have come across it then.
EG, take the drivers kick panel off, trace back one of the Can bus wires from the OBD port (find pin out on here), cut it and extend the 2 ends to a tiny little switch and hide that say, in the passenger footwell under the glovebox somewhere (or wherever you like thats not easy to find)
Just means you need to flick the hidden switch to enable the CAN bus to the ODB, no big deal, a few pence in parts. Best switch would be a latching pushbutton, so you can feel whether its IN or OUT without having to see it.
You would rarely need to switch it on, so you could hide it pretty well in the mass of wiring behind the glovebox.