If you want to nuke trolling, use the Metafilter strategy: new accounts have to pay $5 (once). Troll too much, lose your account and pay $5 for a new one. Hurts a lot more than downvotes.
This will deter some (a lot?) of non-trolls from making new accounts. It will slow community growth. On the other hand, it will tighten the community and align interests. Casual users don't contribute to Less Wrong's mission: we need more FAI philanthropist/activists. Requiring a small donation will make it easier for casual users to make the leap to FAI philanthropist/activists, even if it makes it harder for lurkers to become casual users. And it will stop the trolling.
I think this is a VERY BAD IDEA. Charging $5 would have kept me out. It also keeps out everyone who doesn't have a credit card, which includes basically every high school student.
The recent implementation of a -5 karma penalty for replying to comments that are at -3 or below has clearly met with some disagreement and controversy. See http://lesswrong.com/r/discussion/lw/eb9/meta_karma_for_last_30_days/7aon . However, at the same time, it seems that Eliezer's observation that trolling and related problems have over time gotten worse here may be correct. It may be that this an inevitable consequence of growth, but it may be that it can be handled or reduced with some solution or set of solutions. I'm starting this discussion thread for people to propose possible solutions. To minimize anchoring bias and related problems, I'm not going to include my ideas in this header but in a comment below. People should think about the problem before reading proposed solutions (again to minimize anchoring issues).