It could be solved by small hack in JComments' template:
1. Open /components/com_jcomments/tpl/deafult/tpl_comment.php
2. Find code:
$this->getCommentVote( $comment );
and replace it with:
$acl = JCommentsFactory::getACL();
if ($acl->isObjectOwner($comment)) {
$this->getCommentVote( $comment );
}
In this case vote buttons would be shown for article's owner only.