Online Community of Zenith Builders and Flyers
Although I moderate the forums, I'm not the webmaster and really don't know much about the Ning software that "powers" these forums and don't know how to stop following a specific forum (if anyone has figured a way to do this, please chime in!). However, you can stop following all forums by navigating to the Home page, scroll all the way down to the "Forums" box and select "View All" in the lower right hand corner. Scroll down to the bottom right hand corner of the last box and select "Stop Following" and that should stop all discussions being forwarded to you.
I know that you wanted to just stop emails from a specific forum, but you could still scan the forums you have interest in and always "Follow" specific discussions you have interest in by selecting "Follow" in the left lower corner of the "Reply to This" box immediately under the first post in the discussion.
Victor, with gmail you can set up a filter to send any email with certain keywords, etc. directly to the trash.