As a dashcam are out in the open i dont think you can.
It is the same with other stuff here, if it is not in the boot of your car or in the glove box it will not get covered by insurance if stolen.
You could go with something like the SG9663DR camera, it have its two cameras on wires off a main unit you can hide well in the car, so would take quite a while to take out of a car, VS a normal system where the main unit sit on the windscreen as the front unit and only the rear camera are separated.
Of course if they drive away with your car, they probably have a place to strip it down nice and easy, but smash and grab is the main problem i think.