Courses can be deleted in "Course properties":
Enter the course you want to delete.
Click to "More" in the Course menu.
Click "Course Properties and Features".
Navigate to the ellipsis (three dots) on the top right.
Click Delete course.
Confirm by clicking the Delete button.
Deleting a course will remove the course, all the participants and all the content linked to the course.
Alternatively, you can choose to archive the course by choosing "Move to archive", if you plan to use the course later. When you archive a course, it will be archived for all participants, including co-teachers, in the course.