Reminds me of an issue I've been dealing with. The bluetooth in our car picks my wife's phone every time. Not a big deal, one might think, but then if I turn off bluetooth on her phone, it turns right back on and reconnects! I tried updating the car's settings not to prefer her phone (both were preferred), still a problem.
I believe at some point when we got our new phones, she accepted a dialog that set the car to a "trusted device" which means it will automatically turn on bluetooth and connect as soon as it's in range. But I couldn't find a way to turn that setting off.
Finally I had to force the car to forget both of our devices and I was able to get my phone to connect to the car while my wife was anywhere within a few feet of our garage.