Its a shame about your boa but I have a 6 foot JCP and his typical meal simply does not have what it takes mouth-wise to remove a significant amount of flesh. In fact, when I first got him, I over-estimated his prey size and he did get some big bites. But those were large rats and they never even managed to draw blood. They mangled the scales pretty good but were not able to penetrate. Now, two sheds later, the marks are completely gone. Since then, I've realized that he doesn't need such big meals and he gets either a small or medium rat. Those things simply don't have the biting power necessary to do more than a good scratch.
to the OP:
It is important to distinguish between a mark and a real injury. Minor bites can scratch the scales and pinch the skin underneath.
Also, a bit of neck stretching is fine. Its looks freaky and you can start to see the pink of the skin underneath the scales but its not damaging unless the prey size is extreme.