This snippet will block specific websites and their subdomains from hotlinking from your site.
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^http://(.+\.)?yourdomain\.com/ [NC]
RewriteCond %{HTTP_REFERER} !^$
RewriteRule .*\.(jpe?g|gif|bmp|png)$ /images/goatse.jpg [L]
