One of the reasons scamming is allowed, is because Admin doesn't want to decide what is or isn't a scam. Some cases are very clear, which would be easy to ban, but sometimes there's doubt. The same problem will arise if Mods have to decide which campaign is spamming.
If you ban a false scammer, that is not equivalent to banning a false campaign.
Since moderators already decide what is considered spam via reports, extending that to campaigns would widen the scope to include multiple users, as opposed to individual posts. In the past, some threads have been locked, showing that moderator discretion extends to threads worth of content, some of which may have a greater post count than what was produced by campaign posters in the same period.
Ad-hoc solutions of simply "moving spam" to another place don't work either and result in places like
Investor-based games (99%+ spam to redirect ponzi threads),
Serious discussion (dead board with TWO threads above 100 replies),
Bounties, and
Altcoin Discussion.
Imagine someone was shitting on your floor, and your solution was to tell them to shit in the tub instead. Or, maybe, you decide that you should go upstairs with your actual guests in a new, shoddily-built room. Then, when all their friends come along and increase their shitting output, at least you'll have some refuge from the horrible activity you can never prevent.
At a certain point, you're going to have to ask yourself: are these shit-covered walls worth it to prevent those very few people whom enjoy the smell of shit from being wrongly barred from your house? They have to be, right? After all, we need to maintain those people that are still willing to join the forum in its crumbling yet unfaltering state. Even if 95% of posts are spam or scams, our values are uncompromising.
*
I wouldn't change anything, though. Let's see how this experiment ends, after all.
* subject to potential forum changes
The managers decide what's the “absolute minimum”, don't they?
Look at any bounty campaign and think about the minimum cost required, then think about what their minimum post requirements would be in response to that.