Your concern is valid. But question is why an company hire a campaign manager rather than run by themselves? Because mostly they aren't familiar with forum culture to assure spam free signature campaign. A manager should choose participants who are familiar with his required board during selection. So it will not lead for obvious spam. For example I have ran multiple campaigns which was required post on gambling section. So simply I ignored applicant who aren't familiar with gambling section and didn't post their regularly. I believe always prevention is better than cure.

Anyway if someone found any spammer on any campaign simply send PM to appropriate manager, so they will able to take action against them. Doesn't necessary to create thread for each case.