The Altcoin Bounty section is getting around 17 pages of campaigns updated per day, That is around 680 campaigns on a daily basis that are updated on the thread by means of a last post. The section has 246 pages (some 9.840 campaign entries), being the last listed campaign dates back from 16 months ago.The sheer volume of daily updates make it impossible for a spam-free campaign to be noticed there at all.

I was thinking it over and was wondering if a hybrid criteria could be used as a default sort order. Without going into all the details now, a default sort order could be accommodated to make compatible the co-existence of two types of criteria:
-   last post (current criteria)
-   Total number of merits earned within a specific time period by campaign signatories (i.e. 48 hours).

The idea would be that each page of the section would hold a percentage of posts according to each of the above criteria. For example, the default sort order could still be Last Post (descending), but let’s say 25% of each page was "reserved" for those campaigns whose signatories had obtained most merits while being on the campaign is a certain timeframe.
The result would be that each page would contain 15 campaigns using the Last Post criteria + five more from the sum of obtained merits in 48 hours criteria. Each page would obtain the next batch of 15+5 posts by descending order of each concept.

The above is obviously more complex to implement, but would break the monopoly of last post in favour of campaigns whose members are being merited while bearing the signature.
The values could be parametrized and evolve over time (percentage reserved for merit criteria, time considered), and be complicated further (i.e. take into account currently total earned merit by signatures, with an aging decreasing algorithm).

I’m not sure if it would address your problem specifically though, since in addition to spam-free criteria, ideally the signatories would be people who earned merits in aggregate with some regularity.