I have made it a habit to bounce each and every spam I get to the first 2
hosts in the Receive: list, as postmaster@... and to the sender. The
sender sometimes bounces but I don't care. The hosts which are misused for
this appear to react real quick. As a Linux (and free BSD) user I happen
to know that the mail admin can lock out messages from a certain source
with just a single line of script ;) He can't do that if he is not aware
of being used... Of course you don't bounce the spam to any kind of list
if you want to do this, pick the 2 domains before the last one.

Peter