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).
What if you had to associate your account with a mobile phone number, by getting an activation code by text message? It still has the effect of requiring some real resource to make an account, but the first one is effectively free. There may be some concern about your number being sold to scammers.
If I encountered an unfamiliar blog or forum and wanted to leave a comment, I wouldn't give my phone number to do so, even if it seemed quite interesting. Then I would probably leave the site.