There are several ways that you can stop receiving notifications emails for threads that you've subscribed to in the forums. One of the easiest ways is to follow the - 'Unsubscription information:' instructions at the bottom of the email.
Alternatively, you can click on the icon in the forums and choose 'Edit Options' to change your account settings. Go to the box marked "Default Thread Subscriptions mode" and select an option from the drop-down menu. Scroll to the bottom of the page and 'Save' your new settings.