I use the offline sync setting on windows phone 10 to download playlists over wifi to prevent using mobile data - it seems this doesn't work. Spotify will continue to stream songs over mobile data even if offline sync is enabled chewing up data plans - the only way to resolve this is to switch the app to offline mode. If there is a offline sync version of a song it should not be streamed when played.
Try turning off your mobile data access to the app. You don't download music on data anyways, right? You do it over wifi. So why even have it on? Your mobile data is still going to be on but not for the Spotify app. This is what I did on my iPhone. (Not familiar with Windows phones so I am not sure if you have this option available). Hope this helps dude