A few years back, I tried to come up with a solution. I created Truyoo (http://www.truyoo.com), which essentially requires a user to pay a one-time nominal fee (less than $2) to confirm their identity. They would then be issued a Truyoo ID, which could be used to comment on any site that used Truyoo.
For legitimate, constructive commenters, this is extremely inexpensive -- but it quickly becomes very expensive for spammy/abusive users.
Unfortunately, Truyoo never gained traction, largely because no publisher was willing to ask its users to pay even a tiny one-time fee to comment.