"Unchecking" tracks playing music on iPhone
In the Songs list or Playlist, just swipe it to the left, like you would an unwanted email, text etc

I haven't tested whether this 'reports back' to iTunes &amp; unchecks it there - I'm running a sync now to see &amp; will edit...

Edit:  it doesn't delete or uncheck the track from 'My Music', the definitive iTunes list; but it does remove it from an iTunes Playlist after sync.