They'd just keep creating new keys, and use a botnet to exchange seemingly valid e-mails with itself and a trickle of seemingly legit e-mails (buy access to a few small legit mailing lists to make sure their botnet isn't a total island). It's not clear to me that this won't make it possible for spammers to manufacture/buy sufficient "trust" to put themselves in a better situation than they're in today.
This kind if implies a secondary directory market of ratings and rankings of the prime source (ie isASpammer, isWithinThreeHopsOnLinkedIn)