If you raise the cost of sending a single piece of spam sufficiently, spam will vanish.
Assuming that sending spam costs the same as sending other mail, the cost is limited by what people will bear for their legitimate mail. I claim that spammers can simply steal enough money (possibly out of users' mail clients) to pay for their spam.
AFAIK a lot of spam comes from botnets, which are stolen resources. Also, one might argue that spammers are just people who are too dumb or too chicken to engage in real cybercrime, given that it appears to be more profitable.
Assuming that sending spam costs the same as sending other mail, the cost is limited by what people will bear for their legitimate mail. I claim that spammers can simply steal enough money (possibly out of users' mail clients) to pay for their spam.