Click on Manage Account => Account Settings => Instructors and Users & look for the list of instructors. Click on the instructor name you'd like to delete.
If the instructor is still assigned to a current class time, you cannot delete them. (The class times for the instructor are listed on that same page.) First you need to delete the classtimes (or change the instructor in those classtimes.). Click here to learn how to change the instructor of an ongoing class.
Once there are no class times listed, you can delete the instructor. Previous class history will be saved.