To turn them off, click "Profile" at the top, then "Preferences" over to the right and go over all the options. Find the one that says "always notify me of replies by e-mail" and set it to NO.

I haven't found a way to make "no" the default. We're working on it.

edit by thebornotaku:

and to get it to stop sending you emails for threads you've posted in, once you turn e-mail notifications off, go to "threads being watched", select all and click "stop watching these threads" or w/e.

(thanks kat!)