Hi Cosmo,
If you are serious there are two ways to stop it from connecting automatically. Under your phone's settings -> connected devices -> android auto, you can change the settings in the Start Andoid Auto automatically to "if used on last drive". But every now and then it seems to glitch and get switched back. The other way if you don't want it at all, is to remove the vehicle from the "previously connected vehicles" section of Android auto. I haven't tried that but it should work.