I play Spotify across many devices: On my computer while I work, on my daughter's speaker during bedtime, etc...

Whenever I connect a bluetooth device to my phone, Spotify switches to playing on my phone. 

How can I stop Spotify on my phone from taking over when I connect my phone to a bluetooth device?

For example, I'm at my computer with music playing all day through speakers. Every time I get ready for a call and turn on my headset, music switches to my phone and I have to switch it back after the call. If I put on a bedtime story for my daughter and then put in my earbuds to listen to a podcast through a different app, it takes over her story.