If you've discovered another website using your website's graphics, and you'd like to cut them off, here's how.
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www\.)?mydomain.com/.*$ [NC]
RewriteRule \.(gif|jpg|png|js|css)$ - [F]
and replace mydomain.com with your domain.
You could actually serve up alternate content when hot linking occurs. This is more commonly done with images, delivering an image of an angry man as opposed to the expected image.
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www\.)?mydomain.com/.*$ [NC]
RewriteRule \.(gif|jpg)$ http://www.mydomain.com/angryman.gif [R,L]