Hey girl! I posted about this last week when it was happening to one of my sites...
Just add this in your htaccess and you'll be good... if it's for particular domains that is ....
Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_REFERER} ^
http://(www\.)?stupidfucking theives .com/.*$ [NC]
RewriteRule \.(bmp|gif|jpg|wmv)$ - [F]
Ivy