To delete a teacher go to Employees->Teachers->click on teacher's row. In the options menu on the right side of the "Edit Teacher" page you will see an option to "Delete Teacher". When you click on this option a confirmation box will appear asking you to confirm the deletion.
It's important to note that you can only delete a teacher if the teacher does not have any students associated with their accounts. Any students or events that are related to this teacher must first be transferred to another teacher.