The idea is simple, reconfigure the forum, as well as the merit system, reconfigure it so that if a user does not earn atleast 1 merit in 120 days, that user drops he's current rank for the lower one, and if in another 120 days, the user still does not earn 1 merit, he or she again drops the current rank for the one lower, just like that until that user is back to newbie... This can only apply to junior Members rank and above.
It's not a bad idea to introduce some kind of negative points for inactivity, but you have to think that some good users are not active all the time in forum.
Imagine having some developer who is posting periodically with update of his project, and he needs to upload images.
Now after period of inactivity he is back to newbie rank and he can't post images again, so you would have to separate good from bad users and that is not easy to do.
1. It will drastically reduce shitposting
I don't think it will create any drastic changes.
Spammers and trolls will still find a way to earn one merit every now and end to continue shitposting.