Go to your User CP --> Edit Options --> Under 'Default Thread Subscription Mode' select No email notification or Do not subscribe. This will fix it for future threads. The ones you've already subscribed to may need to be unsubscribed individualy.
To do that go to User CP --> under each thread you don't want notification of, click the 'Unsubscribe' link under each thread. Yes, this is tedious, but you generally only have to do it with the threads that are recently active, as dead threads that aren't being posted to will not generate a notification.