Here, at the top, "preferences" (right under new private messages link), then forum settings/select subscription as default... set it to no.
By default you are subscribed to anything to reply or write on the forum. Setting that to no, will stop it.
As for the existing subscriptions, just pick the thread you don't want to get emails from... at the bottom of the page there is a "unsubscribe" button that does exactly that.

Additionally, when you reply to anything (just try to reply to this for instance), under the text box you'll see a checkbox that says: subscribe, check this box to be notified of replies to this topic. Uncheck it... and you won't receive emails.