From the Staff Members list in the Administration area use the tick box on the right of the staff member to select them - you can select multiple staff members at once

Then scroll up to the top of the screen. Click Options, choose Disable Account, and click on Save.

The staff member is then removed from Backdrop, they are no longer displayed on your staff list and if they are currently logged into the system they are logged out on their next action with the message "Your account has been disabled".