To deactivate a staff member's account you will need to set them to a Deactivated state.
In a deactivated state, the trainer is unable to log in or use the system in any way, nor does their name appear in new client bookings or classes, etc. Their history will be kept and you can reactivate their account at any time.
To Deactivate a Staff Member, go to Settings > Staff > Click on the Edit Button on the Staff Member's Name and Click Deactivate.