by ben reynolds.
For the method above, click the "Turn editing on" button in the upper right hand corner of the course's main page.
Alternatively, in standard theme, click on the assignment, then scroll down to the bottom of all leftside navbars to Settings > Assignment administration > edit settings.
But that's only for an assignment in a course. To disable email notifications for all courses, Site admin > Plugins > Message Outputs > Default Message Outputs > and have at it.