In a novel I am writing, everyone carries a small device in their head. The device is fragile, but the human head isn't.
How can this device be destroyed by another person quickly (i.e. within seconds) and in a realistic way, either with bare hands or some common implement (like a screwdriver or stone) that is easily available anywhere? No guns or other weapons (e.g. swords), as these are illegal.
The death of the victim would be a welcome side-effect.
This is a SciFi story, but the less futuristic tech is involved, the better. I would have liked to destroy the victim's head with a quick, powerful, Bruce-Lee-like punch, but from the research I have done this is not possible.
Since everyone carries such a device (think of the transmitter part of a distributed identity implant), it is built to survive what humans commonly do, including sports (boxing) and light accidents (fall from a bike). To get an idea of the fragility of the device, think of the brain itself. While a brain can easily be squashed with bare hands, you don't usually destroy someone's brain with a kick to the head. The device is like that: easily destroyed, if you hold it in your hand, but cushioned in the brain.
Some notes in reply to some comments:
"fragile": Think of a piece from a plastic bag.
Everyone carries that device. It is implanted at birth (or in utero, if you want). It cannot be heavy or hard, or it would damage the brain if the person fell and hit their head. It is light and flexible. It also must withstand all causes of death that do not destroy the head, because it is supposed to identify the dead person. So it doesn't disintegrate in a car crash. Also, merely killing the person doesn't destroy the device.
"quickly": As my question states, I need to destroy the device within a few seconds.
Or security will interfere.
"inside the head": That's where it is. It's not in the neck. It is always fixed to the same part of the brain, but brains are not all the same, so it is not always in the same place (unfortunately).
It's inside the head because that's where it is difficult to get out of, tamper with, or destroy, without killing the person. It's a security measure, not a technical necessity.