Campaign managers should be accountable for the post quality of these wearing their signature. As per my understanding a certainly *decent* amount of quality is needed in each post to qualify for a payment in signature campaigns.

Shall the manager check each post before payment ? And shall the manager warn/ban these who are not compliant a certain quality (aka Spammers) ?

The answer is Yes. Maybe moderators should warn managers and ban campaigns from the forum if these allow spam instead of removing signature campaigns. Personally I believe these are useful means of advertisement and a powerful tool, the focus should be in preventing quality deviations just for the sake of getting paid.