The main flaw of the ranks is to have focused exclusively on the activity of the users since having a lot of activity doesn't imply that you publish a lot of quality content. In this way, you are encouraging the publication of a lot of content instead of the publication of quality content.
Now that you have implemented the merit system, I would harden as much as possible the requirements for obtaining new ranks by increasing the requirements of merit. For this, you should configure the sources of merit so that they encourage the creation of good content and, in my opinion, also giving priority to other users giving you merit (similar to a +1 system, as mentioned above). During the configuration, you must take into account which sources can be exploited automatically with bots and which ones can not and you will have to maintain a good balance so that the ranks are obtainable but without it being very easy.
Then, the signatures and other additions in the upper ranks would be the reward for good behaviour and for contributing to the forum. Those who want to earn money with signature campaigns should be useful to the forum and contribute, otherwise, the purpose of the forum is completely distorted and it becomes a money-making machine.
- Removing signatures or sig ads globally. Requiring payment to wear sig ads.
Integrating the changes I mentioned above wouldn't require removing them and would be a less abrupt change for the community. I'm curious about how you would implement it to differentiate between users who participate in a signature campaign and those who don't.
Banning account sales.
This should be banned and prosecuted. Many here say that it wouldn't solve anything because, in the end, the sales of accounts would move to other forums. But it's a question of authority and common sense: don't allow in this forum activities that are not wanted.
Merit points would be awarded in a monthly vote on best posts of the previous month, with various measures (TBD) to prevent gaming of the vote. Winning merit points might also come with a BTC prize.
This would be too tedious and wouldn't solve anything: either the restrictions to vote are very soft and the system can be abused, or the restrictions to vote are very high and this becomes an oligarchy. In addition, the "prize" doesn't have to be BTC, it has to be a common incentive that shows the appreciation that the forum makes of the user, without the prize being succulent enough to be worth automating.
Create or designate some sections as "serious discussion" sections, with no signatures.
This would be just a patch, users would stop spamming in those forums but wouldn't cause users to participate in them to create quality content. They could continue to be used to increase activity.
In those sections or maybe in different ones, also have poster restrictions such as Member rank or above only.
Same error as before: a high rank doesn't imply (currently at least) that you post quality content. It would be an entry barrier to the forum and, if widely used, would increase account sales in order to participate in those threads.
And/or allow topic-creators to set these restrictions on their topics, similar to selfmod topics.
If you make signatures and be able to participate in signature campaigns to become a prize, it would take away the advantages of the effort made to achieve it. Also, it wouldn't be necessary because only good content creators and contributors would have access to them.
Maybe at the beginning it will be quite restrictive, but it will make the forum grow healthy.