You have to login to the forum to unsubscribe from a thread you are subscribed to in your control panel.
Go to your User CP, click view all subscribed threads button, click the [x] Notification checkbox in the upper right corner to check all of the subscribed threads, scroll down to the lower gith corner and click the 'selected threads' button > update subscription type > [x] 'Through my control panel only' to disable email notifications on the ones you are already subscribed to.
To have all new threads setup to never email you notifications go to your User CP again and click General Settings on the left side bar under My Settings. In the 'Messaging & Notification' section you will see 'Default Thread Subscription Mode:' that you want to change to 'Through my control panel only'. The default mode is for email notifications since most forum users only use the forums for problems they are having.