Decent campaign managers already don't accept spammers. The merit system even makes their job easier, as they have to check less posts histories.
When it comes to enforcing it, I think theymos chooses freedom over restrictions:
The things on the forum which encourage spam are allowed mainly because it's part of the forum's mission to be as free as possible. Eg. banning bounties would undoubtedly reduce spam, but that'd be destroying an entire economy/population/culture which has been able to develop due to the forum's freedom. I am willing to take this sort of action, but only as an absolute last resort. It's always preferable to handle these problems by reshaping the environment to make them non-problems, rather than removing some freedom.