Unfortunately it's not easy in this version. The next major version is planned to include much more control over who can perform which actions.
In the meantime you can achieve this by changing the following line in qa-page-question.php:
$post['answerable']=($post['type']=='Q');
... to ...
$post['answerable']=($post['type']=='Q') && ($qa_login_level>=QA_USER_LEVEL_ADMIN);