To turn off email notifications or unsubscribe from a thread, click on the "My Account" link and then looks for the "Notifications" tab. There will be several sub-tabs there where you can manage your current subscriptions individually, including a link to immediately cancel all forum notifications. Each email notification should also have a link in it with a link to unsubscribe.
You can also click the "Edit" tab on your profile page and turn off auto-subscriptions to threads you post to and groups that you join.
To manage your subscription to the email newsletter, go to your profile and click on the "Edit" tab and then the "Newsletter Subscriptions" sub-tab.
You can also send an email to [email protected] if you would like assistance managing your subscriptions.