Here is a link I found that might help. distant, early morning - Referral Spamming
I think it is something like;
RewriteEngine On
RewriteCond %{HTTP_REFERER} ^http://(www\.)?spamsite1.com.*$ [OR]
RewriteCond %{HTTP_REFERER} ^http://(www\.)?spamsite2.com.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://(www\.)?spamsite3.com.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://(www\.)?spamsite4.com.*$ [NC]
RewriteRule \.*$ http://www.some-other-website.com [R,L]