When a User has left we are unable to stop them from appearing as available approvers against accounts.
Setting security No_Access. Access level 9, input allowed 'No', Z_LEAVERS user group, Not active and removing them from the Key List of authorisers doesn't not stop a user from showing.

We do not want to delete User records once they have left or become deactivated.

One of the above settings should stop users from appearing in the list of available Approvers.