If a post is really spam or otherwise absolute garbage, you already have available to you a means of not only demeriting it, but asking for it to be deleted: That handy Report to moderator link you see in the lower right-hand corner of every post.
If a post that has been merited is removed, does the user that created the post lose the associated Merits or sMerits? I thought they got to keep both, but that's the only situation were I think the concept of demerit might work well. If the post no longer exists on the forum, it's hard to argue that it deserves any forum merit.