I always thought a simple way to slow spammers down or stop them altogether was to simply require a valid SSL cert, which costs money, to be installed on the mail software in order to be able to send and receive email. Perhaps require a cert per user for individual email accounts or per domain for corporate accounts. I doubt many would complain if each email address had just a one-time fee associated with a cert per email address if it was reasonable like $10-$25 in order to permanently secure your individual email address or email server per domain.
Especially if that's all it took to put an end to spam by pricing them out of business.
For instance I've been using my primary domain and email accounts for over 15 years now so spending a couple of bucks to get a one-time certification of email accounts for my wife and myself wouldn't be much of a big deal really.
Spammers don't like to spend money, nor do they like to expose who they are, so getting certified would be a big issue. They like to leech off the free resources of others and if the only way to send spam is to buy an SSL cert which would be invalidated as fast as each IP or domain caught sending spam was blocked, they'd run out of money real soon. Best part is they couldn't use the same credentials for getting a new cert after being bagged and tagged for spamming.
The downside is that it would raise the spammers desire to hack into mail servers to an all-time high just to use someone else's certs. The other major downside I can contemplate is that spammers would also want to do ID theft to get certs but the upside is sending phishing emails to steal those IDs would be much harder to do if not next to impossible!