Unsubscribe From Forum
You can actually change your email Subscriptions options to No Emails at all and still stay subscribed to the forum. Then just login in via the web and browse messages or create a new topic for discussion. That is also covered in the Forum Emails Frequency FAQ.
However, if you really want to completely unsubscribe, you can do so easily by clicking on the Unsubscribe link in the footer of any email that you’ve received from the forum:
After clicking on that link you will see this message to choose amongst these 4 options:
If you click on the red Leave Group button, you will see this message letting you know that you are no longer subscribed but you will receive one final email with a link to resume your subscription within the next 7 days. That email is shown below this notification.