I've never heard of anything like this, though I've thought of it quite a long while back for Reddit. The problem is that it won't solve anything. Most people will still downvote / leftvote things they don't agree with and upvote / rightvote things they do agree with. If the desire to silence a dissenting opinion could be controlled by the majority of people, free speech would not be such a rare thing, let alone need to be written into law.