Hm. Right now, you can't downvote more than you've been upvoted. Suppose a Plonk costs 1000 downvotes, could only be applied once per user-pair, and increased the minimum viewability threshold of a user by 1. So if two people Plonked timtyler, his comments would start disappearing once they'd been voted down to -1, instead of -3. The opposite of a Plonk would be an Accolade and that would make comments harder to hide, lowering the threshold by 1?

Doesn't actually sound like a good idea to me, but I do sometimes get the sense that there ought to be better incentives for people to take hints.