Hi,
very quick hack: Just replace the check, if the user is admin with a check if the username is the one who should be able to access the admin area:
So:
open /include/staff/header.inc.php, search for
if($thisuser->isAdmin() && !defined('ADMINPAGE')) { ?>
and replace that with
if( ($thisuser->getUsername() = "superuseradminname" ) && !defined('ADMINPAGE')) { ?>
Please note, that i haven't tested that, guess it's obvious that you have to replace "superuseradminname" with the login name of the respective person. And i'm pretty sure, there's a more "elegant" solution as well :)
Best
T.