This will block any referring site with "fusker" in the domain string.
RewriteEngine on
RewriteCond %{HTTP_REFERER} .*fusker*.*$ [NC]
RewriteRule .*\.(gif|jpg|jpeg|bmp|png)$ - [F,NC]
If you want to return a specific image in it's place, use
RewriteRule .*
http://www.whoever.com/your_hotlink_dir/hotlink.jpg [F,R]
I'd use a watermarked image promoting your site. Don't waste bandwidth by using goatse or whatever. I prefer to just drop the request.
Note: on some servers, .* needs to be (.*)
not sure why yet..