How to play single album on Spotify when bluetoothed?
In the Spotify app, go to: 
"Your Library" (lower right) -> Settings (cog in the upper right) -> scroll down and turn off "Autoplay". That should do it. Autoplay does exactly what you said. "Keep on listening to similar tracks when your music ends."

You can also or alternatively turn on "Offline mode" at the top of the settings screen. This will only allow the app to play already downloaded songs and will not use any data. You can leave the app in Offline mode for almost a month at a time. They require you to go back online after 29 days to confirm that you still have Spotify Premium and then you can go back to offline mode if so desired.