What's wrong with charging for an account and enforcing a clear ban-hammer for posts which violate a set of rules? That was SomethingAwful's model and it seemed to work well. Especially combined with their "Leper's Colony" where you could see what each person was temp/perm banned for. That seemed to scale well over time since you have a fixed cost to make an account as long as you aren't getting out of line.

Charging per post does seem regressive and stifles conversation among everyone who doesn't have money to burn / doesn't view the post as an investment.