When not in-use, the Carv tracker is in a low power mode and is designed to not require any user input. If your tracker is flashing blue every so often, it is indicating that it is connected to your phone. 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, you can use the "sleep" option in the Carv tracker configuration screen. The option appears as shown below when tapping the more dots on the top right. To wake the tracker back up again, plug into a charging cable or a Carv Insert.