Question

how to unable users from changing their user names because they can change it from their accounts?

Answer 1

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']),
),