Code for an .htaccess file located in a directory containing images (or any other type of file for that matter) to which you want to block hotlinking from blacklisted websites.

RewriteEngine On
RewriteCond %{HTTP_REFERER} ^http://(www\.)?evilwebsite\.com [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://forum\.evilwebsite\.com [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://(www\.)?bandwidththieves\.net [NC]
RewriteRule \.(jpeg|jpg|gif|bmp|png|JPEG|JPG|GIF|BMP|PNG)$ http://example.com/pwnt.gif [L]

example.com/pwnt.gif

is the location of the file you want to serve in place of the hotlinked image. Mine just says 'PWNT'.

Alternate last line to simply block the images from being hotlinked instead of serving a replacement image:

RewriteRule \.(jpeg|jpg|gif|bmp|png|JPEG|JPG|GIF|BMP|PNG)$ - [F]