Someone with a negative score can't downvote.
What about the dedicated silent downvote-sockpuppets at zero karma?
Still can't downvote. The maximum number of downvotes you can give is 4 times your karma, which is zero with zero karma. So you have to keep gaining karma in order to keep downvoting, and a karmassassination would take a while to save up for.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.