I experienced the same issue on my Kenwood KMM-BT522HD.
I was able to resolve the issue by performing a factory reset on the headunit. When the headunit is turned on after a reset, I was able to adjust the inital settings. I could see in the manual for the stereo that Pandora, Iheartradio, and Spotify were all able to be individually turned on or off.
After turning Spotify off, when the headunit connects it uses the normal Apple USB setting. It is worth noting that it no longer showed any playlist or song title/artist. The factory reset will also wipe out all of your EQ/Presets/etc that you have configured and they will need to be reset. Totally worth it in my book.
The specifics may be different for your model, but hopefully the general process is similar. All in all, it took less than 5 minutes. If you get stumped, look online for the manual for your stereo. That should outline whether or not the app connections can be disabled.
I didn't have to do a Hard Reset, I was able to turn off the Spotify "automatic" connection so now I connect through the generic bluetooth connection. Works well but yes you are correct I don't see any tracks on the display. But I can control using my iPhone. So it works well. I'm actually using it in a boat and I bought the Marine Dash Controller which now only controls the volume, not the tracks but all and all it works.
Thanks for the help.
Re: No control of app while connected to car stereo.
In my case, it was a simple fix. I have a JVC KW-V230BT Stereo that I installed in my car, which supports Spotify Control. All I had to do was enter the system menu, and disable Spotify control in the head unit. This stopped the USB plug-in from recognizing Spotify specifically, and instead leaves the radio in iPhone/iPod mode. I can now use the app while connected via USB.