Got it! Top right of the screen is the notification globe. http://prntscr.com/uww2hx If you place your mouse over it you see a link to 'notification preferences'.
There you can choose what type of activity you are notified about. It looks like you can't 'unsubscribe' from individual threads. However, if you use a bookmark then you can select bookmarked threads to trigger a notification. It would appear that you could then 'un-bookmark' a thread once it has run it's course or you no longer wish to follow it. (I've not tried it so It may not work that way, but logic suggests that is how it will work!).