We really can not force them to do certain things that may lose their interest to advertise in here too. It always has to have a balance and I think we are not doing bad in it so far.
I only remember the Yobit campaign some time ago where they intervened, because the conditions of the campaign led to massive spamming.
Assume that there is no minimum posts requirement. What if there are many users who make only 1-2 posts per week while the campaign needs more active participants?
Be sure managers will likely remove such participants as they have the right remove anyone. So it's better to more clear rule.
I think the better solution is to have a limitation on maximum number posts. (the maximum number of posts that are payed cannot be more than X.)
Well, in theory it's good solution but practically we are very different.
Some users usually only write in a few topics and can't write many quality posts but others have more free time and knowledge so they can write much more quality and informative posts in the forum.
Sometimes we have very interesting discussions that last for days and in which the same users can have many posts.
Not all users are the same, nor are their posts.
In signature campaigns, the biggest responsibility lies with the managers and their selection criteria for campaign participants.
Someone can spam even with 10 posts and someone else can write up to 100 quality posts without any spam, in my opinion.
Limitation on maximum number posts will not help much, I'm afraid.
The best way to stop spam still remains post reporting function.