Remove or comment out the following in qa-page-account.php:
if (!isset($errors['handle']))
qa_db_user_set($qa_db, $qa_login_userid, 'handle', $inhandle);
... and later on ...
'handle' => array(
'label' => qa_lang_html('users/handle_label'),
'tags' => ' NAME="handle" ',
'value' => qa_html(isset($inhandle) ? $inhandle : $useraccount['handle']),
'error' => qa_html(@$errors['handle']),
),