Banning them would be a little bit too harsh IMO, but reviewing each of those posters for a signature ban would be completely fair.
I think it's a great punishment for low-quality posters, it immediately hits them where it hurts.
It is easy to check post quality if one campaign has only 25 or 50 slots in total, but if that campaign opens for 300 slots like Cryptotalk (that figure gave days ago) and will keep rising over days, I think it requires decent time, efforts, and good skills from manager to check, ban bad posters and clean the forum by stopping shitposters earning money from their boring nonsense posts.
Btw, I'm talking about a temporary sig ban, like mentioned in the OP, 30 days should do the trick.
They'll either give up or change their ways, in any case it sends a really strong message to would-be-spammers.
Temp signature ban is just short-term solution, and they will come back with Cryptotalk or other campaigns when their signature temporary bans expired.