[szhu]

Requiring a pin to switch it off might just cause someone to invent a device that can be inserted into the Lightning port to cause a shutdown without affecting the for-parts resale value of the phone. (For example, the device could cause a short circuit or apply an unexpected voltage, causing the power controller to shut off power to the device.) If the attacker has physical access, there's not much you can do.

[paxys]

Here's an easier way - dump the phone in a freezer or ice pack.