"Manage Proxies" functionality is somewhat limited

Why does the "Manage Proxies" functionality under Settings not allow for people to be deleted from the list if they are no longer required to be on it, whether because the period for which they were given the proxy access has lapsed, or because they have become ex-employees?  And how, if an ex-employee had granted proxy access(es) before leaving, can they be removed from the system so that they no longer appear in the select list of the person to whom the access was granted?