Go to your 'control panel', and scroll to near the bottom:
http://www.beginnertriathlete.com/discussion/profile/controlpanel.asp
"Receive e-mail notifications by default?", select 'No'
That should do it....
In addition to that, any threads or forums you're currently subscribed to may need to be manually unsubscribed. The links to do this are in your Control Panel at the top of the page.
Unsubscribe from threadsMass subscribe / unsubscribe to forums