Thanks Gideon! Here is what I did in case anyone else wants it:
Open qa-app-users-edit.php for edit
After this line:
$errors['handle']=qa_lang_sub('main/max_length_x', QA_DB_MAX_HANDLE_LENGTH);
Add the following code:
elseif (preg_match('/admin|moderator/', $handle, $matches))
$errors['handle']=qa_lang_sub('main/word_not_allowed_in_username', $matches[0]);
Now, you need to make an error message for it. Open qa-lang-main.php for edit.
Add something like this to the array:
'word_not_allowed_in_username' => "Sorry, the word '^' is not allowed in the username.",
That's all.