Demerits is a very interesting concept. That would be similar to a dislike or a downvote but it would have far more lasting consequences because it could effect a member's ability to use signatures, personal text, etc. I think it would incentivize higher quality posts as well and would potentially reduce spam even further.
However, like with most systems there are some that could also abuse this feature. Specifically if a member doesn't agree with another member's post they could simply use demerits to penalize them. Or they may choose to demerit another member out of malice or simply for entertainment.
Unfortunately, no system is perfect but i think right now the forum is taking the right steps to control spam but they will need to collect data and see how lasting the effects from the current rules will be before deciding on any further changes.