The 'emergency' answer:
'Quick Links' -> 'Subscribed Threads'
Click box next to 'Notification' on [ blue ] bar at top of thread list.
Click drop-down menu button 'Selected Threads' at bottom of list.
Click 'Delete Subscription' and then 'Go' button at bottom corner.
You will have to page further down in the list to do the same to more previous threads...
Also:
'Settings' -> 'General Settings'
Set 'Default Thread Subscription Mode' to 'Through my control panel only'.
Then, the 'default' will be "no subscription"; however, you can set the subscription of each thread individually using the method above - clicking the box next to the thread in the list instead of the one on the "top bar"...
EDIT:
"Kinda what Astrid said, but with a greater sense of 'urgency' and a larger 'scope'..."