How about something like
1) look from which IP address the spam came in
2) ARIN, RIPE, etc. send a bill to the owner of that address group for
each confirmed spam incident
3) If the owner of addres block does not pay up, address block is
revoked.
Some clarification:
- the owner is free to the bill to the real culprit (and add some
extra for the trouble), if it knows it. Should be some incentive to
keep their own house clean and neat.
- it does not matter whether source is relay or not (incentive to
close open relays)