As a safety feature there is no power key in device.
However it's important to power off the tracker when entering into places where usage of mobile devices is forbidden, like in airplanes.
Power off functionality is only available from device owners account, not from guradians' accounts.
To power off the tracker:
1. Go to tracker menu from the main menu
2. Choose settings from top right corner
3. Select 'Power off tracker'
4. Press the button on your tracker
You will receive a "is thinking about you" message to your application followed by confirmation that the tracker is now powered off.