The page you linked says “If the monitor is set up on the AirGradient dashboard, it will also receive configurations from there. In case you do not want this, please set configurationControl to local. In case you set it to cloud and want to change it to local, you need to make a factory reset.” So there is a secure way already, but then there should be a server giving commands to the devices.
For my case it would suffice to have configurationControl option none, so that settings would be set once and then only hard reset would work for re-setting them. Or then just a simple password, so the request would be something like curl ... PUT ... http://.../config/mysecretpassword. Not a perfect solution of course.
However, to clarify: If I put configurationControl to cloud and postDataToAirGradient to false, how much unnecessary network traffic it will make to Airgradient server?