It is expected behaviour for the previous track to continue playing when you click play on the next one. You must eject the previous track if you want it to stop early.
I believe there was a bug in a previous version where the previous track would stop when the next track was played, but this was not intended behaviour.
Oh! That's a shame, I quite liked that bug! Thanks anyway.
Ooh Err
So I have the programme set to Live Assist mode, with the segues set to "next" instead of "stop" - but when I press play on the next track, the previous one continues playing until I manually eject it. It never used to do this, so I don't know what's changed to make it start. Any ideas on how to stop it doing that?