You can stop others from hotlinking your site’s files by placing a file called .htaccess in your Apache site root (main) directory. The period before the name means the file is hidden, so you may want to edit your file as htaccess.txt, upload it to your server, then rename the txt file to .htaccess in your directory. Contact your web host on how to access your directories and configure your .htaccess file.
Example: Your site url is www.mysite.com. To stop hotlinking of your images from other sites and display a replacement image called qX4w7.gif from an image host, place this code in your .htaccess file:
[bash]
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^http://(.+.)?mysite.com/ [NC]
RewriteCond %{HTTP_REFERER} !^$
RewriteRule .*.(jpe?g|gif|bmp|png)$ http://i.imgur.com/qX4w7.gif [L]
[/bash]
From:
http://altlab.com/htaccess_tutorial.html