Positive reinforcement. Theymos (or someone good in his name) runs a campaign for the board in addition to usual ads. Exceptional rates (compared to other campaigns) but very strict anti spam policy. This would create a positive example for others to follow or to work towards. This is already given in a sense by other well run and high paying campaigns. It is also not unheard of that exceptional and highly active posters can negotiate special deals.
i like this idea quite a bit; positive reinforcement for good behavior has its advantages in the long run over punishment (negative reinforcement) for bad behavior; psychology and stuff. The only problem i see with this is the demand to be included in this exclusive group. to maintain this exclusive campaign, appropriate amounts of manpower would be needed to constantly monitor the posts of those enrolled. not only that, with people that would constantly be requesting to be included in that group, manpower would need to be dedicated to screen those as well. to top it off, as we know, spammers are quick to adapt, and may do so just to have a chance of being enrolled in these exclusive campaigns. but then again, that would be fixing their spamming problems in a way.
The silent treatment. A silent ban or mute. The spammer can still post normally, but somehow everyone is ignoring them. The posts are not actually shown to anyone but the person with the silent ban. Probably a technical issue as I doubt it is already implemented. Its common for video games to keep players on the server or in the game without causing further issues due to chat/mic spam. Probably not worth it, considering that its easy to check with another account or with a simple logout.
on the other hand, i dont like this one quite as much; even if those posts are "hidden" for the rest of us, that data (for the posts) has to be stored somewhere, and it might cause problems when the user's "silent ban" is lifted, and then suddenly we see a flood of posts in meta along the lines of "my posts arent showing up on ___ account help" or "my account is bugged ive sent theymos / badbear 500 pms no reply help." maybe if all those posts were deleted at the end of the ban period? idk. seems like a headache to implement though.
besides, that idea is just an enforced ignore; if other users were annoyed by the spam already, they can just ignore the user and not see their posts.