Its better to have a fixed rate for sig campaign per week like what ore-mine.org does, with fixed rate spammers will avoided too much bump, thank you, and many spam posts.
The main reason for so many spams is due to automated signatures and even worst if there's no limit in postings. Shorena is right. However, fixed rate campaigns does lessen the load on the manager when counting posts. They should review each participant individually and ensure that they aren't spamming and is contributing something to the discussion at the very list. I have joined many campaigns which only recruits high quality posters and it does work well.
You didn't understand what shorena said. It is not the limiting of post, she is referring to the fixed rate for lower ranked accounts, people tend to spam to raise their pay rate so thus they will make a lot of post and thus that will result to spam.
Also it is not the automated counting that does the problem. It is not a problem to have a bot to count the posts but I think implementing a rule that a participant must take care of the image of the campaign might lessen spamming, let's say there is a rule that if you want to get paid, you need to be sure your posts are constructive, if you want to get paid you need to take care of other participants, such reporting them if they are making spammy posts. Participants should be given a responsibility like this.