Add the following to your .htaccess file to block posts to wp-comment-posts.php and wp-login.php that have do not have an http referrer or user agent. This will also redirect the user back to their IP address.
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{REQUEST_URI} .(wp-comments-post|wp-login)\.php*
RewriteCond %{HTTP_REFERER} !.*domain.name.* [OR] …