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.
I know that ibminer and LoyceV have already answered your substantive questions; but I thought you may find this interesting, as to deleted posts generally: Somebody
traded merit between alts and then deleted the posts, evidently hoping to cover his tracks. The merit is still there, though I wonder why all three involved accounts are dripping red.
I think its one of those instances of any system necessarily being imperfect if it involves human beings. On balance, I think its much more important that legitimate users keep their merit for deleted posts, for such reasons as noted by LoyceV.