The less offensive form of this kind of hotlinking prevention is to provide a small-sized image with the URL of the website it was taken from, or a hotlinking notification, not pr0n unless you want to offend unsuspecting visitors. The person that hotlinks an image for usage as a forum avatar won't even realize it most of the times because his browser cache kicks in.
IIRC, other users on the forum started complaining that the offending user was using pr0n, he (or the forum owner) then complained to the person from whom content was being leeched who promptly told them to suck it.
