If you are not sure what referrer spam is you may wish to check this article:
http://en.wikipedia.org/wiki/Referrer_spam
To block these spam referrers you need the following lines in your .htaccess file and replace example.com with your domain:
# BLOCK NO-REFERRER SPAM - Simply block the request
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{HTTP_USER_AGENT} ^$ [OR]
RewriteCond %{HTTP_REFERER} !.*example.com.* [NC]
RewriteCond %{REQUEST_URI} /wp\-comments\-post\.php [NC]
RewriteRule .* - [F,NC,L]
</IfModule>
# BLOCK NO-REFERRER SPAM - Redirect to a webpage
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{HTTP_USER_AGENT} ^$ [OR]
RewriteCond %{HTTP_REFERER} !.*exmaple.com.* [NC]
RewriteCond %{REQUEST_URI} /wp\-comments\-post\.php [NC]
RewriteRule .* http://example.com/ [R=301,L]
</IfModule>
