Inactive employees can be deactivated, and then reactivated again if need be.
This can be done by finding the employee on the training administration page. Click on the employee, and then in the information that opens up regarding that employee, you will see a button that says Deactivate Employee.
On the same page, under the big button that says Add New Employee, you will be able to switch between active and inactive employees. Inactive employees can be reactivated by following the same directions.