I have been using Sonos with Alexa and Spotify for ages now.
When I say "Alexa, play Stairway to heaven", it plays it just fine on my Echo, then stops. (no autoplay)
When I say "Alexa, play Stairway to Heaven in the Family Room" (I have a Sonos called family room, but this happens on any of my Sonos devices), it plays Stairway to Heaven fine on my FR Sonos, but then continues to Autoplay songs after that. For some reason, when I control the Sonos through Alexa/Spotify, it controls Spotify with the autoplay feature on. I have disabled Autoplay on Spotify on both mobile and desktop.
How can I disable Autoplay when using Alexa to launch a Spotify song on my Sonos? Again, everything is configured properly and the Sonos skill works fine. Just have to turn off autoplay.