Have in mind that IF you have a redirect at the end of the htaccess, that page will load instead.
An example:
They have fuskered your domain.com/01.jpg image that is 30k. You put the htaccess in there and redirect them all to your LL that is 150k (HTML + images). So what will happen is that the request for the 01.jpg redirects and loads your LL instead. They will not see it since it's "trapped" in a image, it will only be seen as a broken image.
What I said above hopefully makes sense after you read it couple of times, I just typed it up real fast without fixing bad sentences
So instead of this line in the htaccess
RewriteRule (gif|jpg|png|swf|mpg|avi|GIF|JPG|PNG|SWF|MPG|AVI)$
http://www.yourdomain.com/bad.html [R,L]
use this:
RewriteRule (gif|jpg|png|swf|mpg|avi|GIF|JPG|PNG|SWF|MPG|AVI)$ - [F]
That way nothing will load and they will get a forbidden error = no bandwidth used.
Btw. post what your htaccess looks like