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 …