Click on the Einstein-looking logo on the bottom right of your screen then select "Staff". If the professor is assigned to a class you need to "Unassign" him/her first. After unassigning the professor, back out of the Hire/Staff settings then head back in to "Staff". Click on the professor(s) you want to get rid of then click "FIRE". The "Not Assigned" professors can be fired and will be located above "Assigned".