You can leave a course by navigating to the course's dashboard and clicking Leave Course, which is located on the right-hand sidebar.
You must confirm that you want to leave the course by clicking Leave again.
After leaving a course, the course name will disappear from under Current Courses and you will no longer receive email notifications related to the course.
Word of Caution
Upon leaving a course, all of your associated coursework submissions will be deleted as well.