I believe there is a really simple solution to this. Spotify is only using your network bandwidth to serve your music files that other people want to play. If you don't want to participate move your music folders to another location that Spotify cannot find.
Interesting, I assumed that Spotify just distributes data that originated on its own servers. If distributes music I've ripped myself, I could screw up its service for other listeners. Does anyone know what the behavior is?