Currently, Spotify does not have any way to stop playback after current song ends. There's no way to play just a single track ONLY ONCE in the playback queue and stop after it. Some suggested work arounds by other people don't seem viable.
I've seen and used this feature since a long time in a not so popular player (foobar2000).
Stop after current allows a user to maintain their playback queue ven after current song finishes playing (possible scenarios: you want to listen to this last song before you start studying/go to sleep etc.)
Current solutions to play a single track are to either remove all other tracks from the playback queue or maybe just have one track in an album.