At the moment, encrypted spam doesn't exist. But only because there aren't enough people using encryption to make it worthwhile for the spammers to invest time doing it.

Theoretically, a spammer could scan the public keyservers for email addresses and public PGP keys and then send encrypted spam to all of those people. If ever a well targeted spam mailshot was sent, that would be it. But would you really want to get on the bad side of thousands of tech nerds?