Sorry I didn't reply earlier.
Go to User CP > Edit Options
In "Messaging & Notification" - "Default Thread Subscription Mode" choose an option you want from drop-down menu ("do not subscribe" if you don't want to be subscribed automatically). Scroll down to the bottom of the page and click the "Save changes" button. Done.
But you are still subscribed for some threads. You need to unsubscribe from them.
User CP > Subscribed threads > click the link "View all subscribed threads", mark box(es) to the right from the thead(s), choose "delete subscription" from drop-down menu, and hit the button "Go". Done
