The root problem is this:
The merit system appears (from what I can tell) to have been implemented after signature spam had been a problem for a long time. Prior to the merit system being implemented, there were many accounts with a high ranking (including many 'farmed' accounts) that, even today have not produced substantial content. These accounts were grandfathered into their rank when the merit system was introduced.
These so called "zombie" accounts have continued to both post nonsense posts and are participating in signature campaigns such as YoBit and many altcoin campaigns that do not care about post quality/substance.
Proposed solution:
Partially remove the rank grandfather --
If an account was a Full Member immediately prior to the implementation of the merit system, they will need to receive 1/10 of the merit necessary to achieve each rank, up to Full Member, and after they are a full member, they need the full amount of the merit.
As an example, someone who was a Full Member prior to the merit system being implemented, would need 1 merit to become a Member, 10 Merit to be a Full Member, 160 to become a Senior Member, 390 to become a Hero, and 990 to become a Legendary member. To explain, the first 1/10 of the 'air dropped' merit the person received is worth 10x, and each subsequent merit the person receives is worth 1x. This would only be for ranking purposes, and not for trust system purposes.
As another example for someone who was a Senior Member immediately prior to the merit system being implemented, they would need 1 merit to become a member, 10 to become a Full Member, 25 to become a Senior Member, 275 to become a Hero member, and 775 to become Legendary.
If this is too controversial, everyone could retain their publicly displayed rank, but those who are demoted, would only lose their signature features.
It's a better option to disintegrate merit with time. Take away 1 merit for every 100 posts. That way the user would be concerned about making good post to hold their status.