I get your point and the idea your postulating but, with the fact that, posts are what actually gets these merits and the ranks just comes as what is been conferred on a user based on their merit and activity accumulations, it feels odd that there should be a demerit system. It’s you indirectly saying, merits should be taken off a given post because, you’re not actually taking merits from the user, the user was never merited but, the user’s posts was.
How will you come to terms with that?
Perhaps if there should be any of such demoting system, I would say we already have it few forms.

Like we have in the wall of shame thread,
We also have DT members to note a few trash posters with tags,
Some campaign managers have got a blacklist for users that are suspected to fall in this category,
We also have the ignore button to keep them off sight.

Again, your rank doesn’t mean you know so much. Ideas are limited based on exposure and what niche you frequent. Not sure we need a demote system, it wouldn’t do any good.