Maybe some campaign managers feel pressured in to filling slots therefore allow many things to take place which they normally would not. It depends on the campaign manager and also on the business being promoted. I agree, lowering payment and bonuses will probably bring an even lower quality of post from spammers but there is no easy solution except to self-moderate threads in boards where signature spammers and scammers are running riot.
I would rather emphasize here the responsibility of campaign managers who tolerate this type of spam posting. Most campaigns are run by casinos and gambling sites and it is to be expected that they want their signatures in the gambling section, but various bonuses only encourage additional spam. A lower payment rate will bring the lower quality of signature posters, in the end, less effort from the manager himself to check every post from every participant.
The gambling board is starting to look more and more like the bounties section which is completely moderated by the campaign managers.