The idea of up/down voting expressing agreement/disagreement seems standard. At least, that is what I have seen. If we try to impose a new standard (downvote -> moderation), then users will (probably) ignore that standard or not follow it.

Therefore, why don't we try to do something different. Instead of having upvote/downvote arrows which implies the standard, we can use a plus sign (or an arrow up) for agreement, and a null sign or an 'X' for disagreement. That way, down voting might not imply disagreement, because an 'X' does not mean disagreement; it means 'not allowed'.