A capped amount BTC will do the same as linking payments to activity instead of posts. Both Bitmixer and Bitcoin Scratchticket campaigns have caps on the amount they will pay for posts - 0.035BTC, Bitmixer and 0.010BTC, Bitcoin Scratchticket - thus limiting the incentives to spamming posts to the moon. Problems with spam come when unscrupolous campaign managers tell enrollees they will pay posts even over maximum limits like Bit-X formerly did.
This is a good point that the max payout does something similar to linking to activity however, in my opinion, linking to activity was even better because as an advertizing poster, you didn't need to post anythign at all in order to get a little payment. Therefore there was no incentive to post anymore than when you actually had something to say. The old bitmixer campaign was a model situation, in my opinion. I guess at some point they must have felt like they weren't getting their money's worth as advertizers, though, because, alas, things have changed.
Bitmixer seems really concerned about the need to limit spam. IMO another good practice could be forfeiting sig ad payments to spammers and giving them to initiatives like Help the forum and earn Bitcoins!. Something like a Pigovian tax to curb spam - spam can be considered a negative externality.
I would like an activity-linked payment system, too; however, i'm not sure advertisers find it good enough for them.