In another thread I already proposed a possible solution: making it possible for users to
ignore entire threads.
That would not "clean" the forum instantly, because these "spam megathreads" could be opened and answered to without any restriction. But first, it would simply be an usability-boosting feature as users simply wouldn't see the ignored threads anymore.
Still more important perhaps, it would create incentives for managers to not pay out posts in these threads. As threads that are ignored by many users wouldn't have many page-views, companies wouldn't want to pay users for signature campaigns where most posts are accumulated in these sections.
I know that signature campaigns with ICO tokens are very cheap for companies, so in these cases the incentives to control quality are weak. So maybe another part of a solution is to require signature campaigns to pay out at least a part in Bitcoin (or another established cryptocurrency) - this was already proposed somewhere.
Restricting Newbies to create threads only in Beginners & Help could maybe also be part of a solution. While it wouldn't stop the spammers to rank up to Jr. Member, like The Pharmacist wrote, it could simply make the rest of the forum a bit cleaner, as a spammer would risk to get banned with an account where (s)he already invested some time in if he wants to open pointless threads in other sections.
I think the most easiest way to deal with that problem is by not paying for the posts in the redundant threads.
Quality control is already done by some managers. The problem are those (mostly altcoin/ICO campaigns) that don't do it. In my opinion, some restrictions are necessary, as I wrote above.