Not that I know you have any volunteers but have you considered having a "positive" moderator just for this forum?
Basically someone you can trust, who can come in and edit out abuse from posts, send warning PMs and do small time temp bans (24-48 hours) with an ability to pass up the food chain to a Full Time mod for a long time ban if the poster continues.
As long as you trust the person not to abuse the very small amount of power they have, it would force people to behave, and in the long term I believe the forum would be self regulated by the fact that posters understand here more then anywhere they would be censured for their action.
This is not my first forum, and I not read enough here to say that things have not got better since you first posted the OP. I do know that there are other boards out there that have political debate with very strict rules about personal attacks and abusive language, and they do still work. Though they are very actively moderated. I am sure some smart alec will tell me to go post there then but it isn't a comparison for criticism its a comparison to show it works, that it can be done....besides that Board is very right wing and it gets boring arguing with the same people.