I believe in prevention being better than cure but your logic is that you need to stop spammers rather than preventing them from spamming. So well it's like wait till they spam and then ban them but in this way, the campaign's reputation suffers saying that it has a lot of spammers.
No it isn't. If you read what I said you would see I suggested doing both. When people sign up check their posts - if they're crap they get denied straight away. Then keep an eye on them throughout the campaign and if they start spamming warn or kick them off.
Are you saying each thread just needs one signature impression then advertising objective achieved? Not at all. The ads actually work better when there's several, just don't pay people to make poor posts, pay them to make quality ones and not the opposite as you're suggesting.