You can also hold down the Home Button to activate Siri, and say Stop. If the Music app is playing, it will stop.
Alternately you can say Play, and the Music app will start playing again where ever it left off last time. If the app was cleard from RAM (so it doesn't remember what you were playing last) it will play your entire music collection, shuffled. If you add Shuffled to any play command (artist, playlist, genre) it will shuffle them.
As near as I can tell you can say "play the song song-name" and it will play just that song, once. Provided only one track matches your search.
I'm going to test that last to be sure. If I don't edit this post it means it worked as expected.
Edit: I was wrong. It went on to play another song. I'm not sure what criteria it used, but the song was from the same album, so next song on the album is the most likely.