IMO it's not about what type of campaign is run, but how the campagin is run by the manager.
I've been managing both, and I think both sort of campaigns can be successful, without either of them spreading spam around the forum, if managed correctly.
What you need is campaign managers paying attention to the quality of posts, which right not they have no incentive to.
Please don't generalise, your statement is wrong for several reasons.
First, there are managers who pay attention to their participants, some more close, some more vaguely, but saying they don't is a front against all of them.
Secondly, there are enough incentives for a manager to keep an eye on the users and their,
as those users represent the service they advertise and you wouldn't want to be advertised by some kind of users, believe me.
If someone is in a pay per post campaign then he will try to post the maximum number of posts counted in that payment period,hence creating spam.
You're asuming a certain kind of mentality for every user enrolled in a campaign, which they don't necessarily have.
But in a fixed payment campaign, the person only have to post minimum number of posts required for the payment, as more Posts will not pay him, so he will not spam.
Asuming X posts in a term of Y days can't be spam, just because the minimum number in a fixed campaign is lower than the maximum number in a ppp campaign.
The quality of someones posts isn't determined by how many they write in a certain time, but by the actual content of said posts.
I can show you users with ten + posts a day, who no one would consider spamming, and I can show you users with one or two posts a day, who clearly are sig spamming.
What are your opinions about this proposal.
Nope. Nope. Nope.