I moved your post over to the Automotive section of the community. At this time, there isn't a way to disable the accessory mode.
You can still listen to Pandora in your car by connecting your mobile device via AUX cord or Bluetooth, rather than using the interface on your dash.
Please note: If you launch Pandora through your Apple CarPlay system, you can use Siri to control what you listen to. If you'd like to have the ability to disable accessory mode, I would recommend checking out this.
If you'd like to ability to disable accessory mode, feel free to create a feature request here.
Hope this info is helpful. 🎧
*Use your mobile device only when conditions allow and as permitted by applicable law.