Preventing Hotlinking Of Images

The images I am trying to block are on page generated by a simple PHP script on my server. The offender has replicated what I am doing with ASP on their server. They are hotlinking to my images for the resulting page. They left my website's name on them, so they must think that giving credit is enough.

I'm going to be contacting them to stop but I also want to see if there is a way for me to prevent it from happening in the first place.

I tried mod_rewrite...

Code:
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^[url].*$ [NC]
RewriteRule .(gif|jpg)$ - [F]

I know mod_rewrite works on my server because I've been using it for some other things.

However, whenever I enable the above code (add it to the directives and restart apache - have also tried just putting it in a .htaccess file in the appropriate directory), I end up with images still being allowed on my domain and the other domain I'm trying to stop from using my images. Do you think it could have to do with an absent referrer? I read that the code doesn't work if the referrer is blank. What else would cause this not to work? Obviously the domain would have to be correct, but it doesn't block from my domain OR the offending domain.

I tried another method:

Code:
<FilesMatch ".(gif¦jpg¦png)$">
SetEnvIfNoCase Referer ^$ allow_image
SetEnvIfNoCase Referer ^[url] allow_image
Order Deny,Allow
Deny from all
Allow from env=allow_image
</FilesMatch>

This one blocked images to the offending domain, but it also blocked mine!