http://www.greenguysboard.com/board/...em#faq_hotlink
or, you could do something like:
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://www.canadafreeporn.com/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://canadafreeporn.com/.*$ [NC]
RewriteRule \.(gif|jpg|js|css)$ - [NC,F]