You can find your subscribed items from the Workshop page. On the right-hand side, select where it says "Your files" and choose "Subscribed items". This should show you a list of all the workshop items you are subscribed to.

Then you can simply click the "Subscribed" button to unsubscribe from the item. Steam will remove the track from your game after some short delay.

In future, we hope to have a way to do this in-game directly.