Was able to force use of integrated card by switching it to "integrated only" in gfxCardStatus and starting Spotify. By checking Activity Monitor i can see that integrated card remains in use even after spotify was started (worth mentioning, spotify started with no issues whatsoever)
Same issue here: every time I open Spotify the discrete GPU (AMD Radeon Pro 460) gets enabled.
MacBook Pro (15-inch, 2016)
I've asked support via Twitter and first they suggested to try disable Hardware Acceleration (it's not an available setting in Advanced), then they checked my account (why?), then they told me to reinstall (why?). None solved it of course.