Just thought I'd mention that there is a return proposal amongst the proposals. What it does is set a monetary bar over which proposals must reach before they are accepted. It's sort of a blanket downvote for all proposals.
It's not exactly what you are talking about, but it does vote against all proposals, so it does some of what you seek.
Right, it isn't. I voted for the return proposal long ago, because it's the best we have. I'm for keeping the Return Proposal in place while also adding downvotes.