Android - How can I stop music apps from playing when my Android connects via Bluetooth to my car?

There's a Bluetooth connect & Play app that can intercept the incoming "play" command. While it was originally designed to let you start a specific music app playing when a specific Bluetooth device connects, you can instead select "STOP music from playing" to keep any app from starting when the car connects.

Go to Settings, Applications, Application Manager, and then Modify System Settings. Find the player that is starting (for me it's Samsung Music) and switch the toggle to OFF.