I'm afraid moderation (of questions, answers or comments) is not yet implemented in Question2Answer.
You can approximate this behavior by modifying the code to automatically hide comments when they are first posted, then using the 'Hidden' panel of the 'Admin' section to find and show those posts if you approve them.
To do this, modify the function qa_comment_create(...) in qa-app-post-create.php by changing:
$postid=qa_db_post_create($db, 'C', $parent['postid'], ...
... to ...
return qa_db_post_create($db, 'C_HIDDEN', $parent['postid'], ...
Please note that this will also stop notifications being sent out when a comment is posted. Fixing that would require a larger architectural change.
