You do not need to turn Carv off. When not in-use, the Carv tracker is in a low power mode and is designed to not require any user input to conserve power.
If your tracker is flashing blue, it is connected to your phone. Simply, closing the Carv app on your phone will disconnect it. When you open the app again, it will automatically reconnect.
If you really want to switch Carv off (e.g. over the summer - although this is not essential), you can use the "sleep" option in the Carv tracker configuration screen. The option appears as shown below when tapping the dots on the top right of the screen. To wake the tracker back up again, plug into a charging cable or a Carv Insert.