Google could fix this rather simply. Have an opt-in system whereby anyone sending you email is charged a penny. (Of course, this will initially mean the sender will need to get a google account, but as this catches on more email providers will provide this service.) Split the revenue between the recipient and Google.

I don't mind spending a penny to send an email, I don't send enough to make any difference. But to spammers, it makes abusing the email system unprofitable. And if I receive half the revenue from people sending me email, it'll likely work out to costing me about nothing.