It seems like the actual problem here would be solved, like many of the forums problems, by eliminating signatures. No signatures, greatly reduced incentive to spam, no need to have bots deleting suspected spam.
I don't think so. Most of the campaign managers (at least nowadays) have a minimum post character requirement and stuff like "Thank You", "Thank you for organizing this " doesn't get paid for. So if someone is actually doing it, it means it's either out of their will to genuinely thank someone or they belong to a campaign that is poorly managed. So the problem boils down to not eliminating signatures but eliminating bad managers who run the campaign.