If you have negative karma and can't post, this is because WE DON'T WANT YOU TO MAKE POSTS. This is as it should be.
So far, I very rarely see anyone making posts, getting downvoted, changing accounts, and making more posts on the same topic. As sockpuppetry isn't that hard to detect, I think we would have a good idea if this was rampant. This would have to be an actual problem before I removed something I want from the forums to fix it.
The only case I can think of is that Dmytry/private_messaging guy who went through at least three accounts to keep on ranting and ranting about god knows what. I don't know whether he stopped posted altogether or changed his behavior so as not to be such a nuisance, but in either case, mission accomplished!
It came to my attention that when you receive downvotes for your comments, your karma goes negative and you need to "pay back" to be able to post to Discussion or to Main.
Since new users start with zero karma, having negative karma seems to just encourage those with negative karma to create a new account. We don't want to encourage people to create superfluous accounts, do we? Therefore I think LessWrong codebase should be patched so that karma does not go below zero even with lots of downvotes.
What do you think?