Options +FollowSymlinks
#Protect against hotlinking
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www.)?domainname.com/ [nc]
RewriteRule .*.(gif|jpg|png)$ http://domainname.com/img/stop_stealing_bandwidth.gif[nc]

http://www.polepositionmarketing.com/seo-sem/link-building-secrets/maurizio-petrone.php

Tired of people using your bandwidth by putting the images hosted on your server on their website? Add the following code at the bottom of your .htaccess file to prevent hotlinking.

Prevent Hotlinking

htaccess

Apache