Another part of the general spam problem is that spam blockers do not keep spam off the network and do not cause the spammers any pain. Most of them block spam at your machine, or at one point upstream from your machine. So the spammers just try harder to get past them by sending more messages, and that further congests the network.
One scheme that seemed to me to have some promise was to detect spam in the SMTP receiving program and deliberately delay its responses to the sending program. So that the transaction of sending a message is stretched out far longer than normal.
jhhaynes at earthlink dot net