Block Unwanted Referring Domain Traffic - Block unwanted visitors based on the referring domain. You can also save bandwidth by blocking specific file types such as .jpg, .zip, .mp3, and .mpg from specific referring domains.
# Block visitors referred from indicated domains
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{HTTP_REFERER} scumbag\.com [NC,OR]
RewriteCond %{HTTP_REFERER} wormhole\.com [NC,OR]
RewriteRule .* - [F]
</ifModule>