Spam can clearly be seen as spam if you look at the names behind those spamming points that repeat themselves constantly. It won't affect the general rating of a post if most of people understand that "player X obviously dislikes player Y and is constantly neg repping player Y on new posts". Furthermore, if the amount of total rep difference is not shown on a forum profile, then spamming someone doesn't really have a point, it has more chances to backfire because the society should figure out player X as a player that cannot weigh posts of others properly, and he will lose a percentage of his credibility.
If the points are already shown, then the spam does work in the interest of the spammer, because the spam point hides behind the number.
Touche on the weak minded part, but not all of us are totally independent of cunning subconscious messages, and we are trying to minimize the distortion of an image a random post has.
After all, it's about who dislikes it, not how many dislike it, I thought MD prefers depth, goes against the mainstream and hates democracy :))
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to help visualize the first part, imagine you, dst and eon neg repping dark demon's posts in shifts, so his every post has "by default" -2 or -3 points. A random average dude will be more quick to disregard his post because he will not bother a lot to see who liked/disliked it. But if he notices it is the same group of few people repping his posts, he will be more prone to disregard the mentioned reputation points.
Of course, we can say that a person should be able to individually rate a post, and not be radically affected by those reputation points, but those red and green numbers are direct suggestions for you to be more (or less) approving of a post, let's get rid of that trick.
Just keep a number of the amount of people that repped the post as a default value, to signify whether it is a popular post, rather than implying it is a good or a bad post.