There are 2 ways of doing this. Manually on each thread - Once You Click On Advance Button and there you would see a combo box which you can change for individual subscription. Automatically to all future threads - In UseCP Visit Edit Options and you would see there "Default Thread Subscription Mode" and you can edit that so you don't subscribe automatically but can do so manually to any thread if you want to. Unsubscribing to all previous thread can be done in UserCP or even in the mails which you get for the thread subscription.
