I know there are forums out there that require you to pay a sign-up fee to either post or browse, but what about a Pay2Post system?
The forums would be free to sign up for and browse, but users would have to purchase X amount of credits in order to post. Credits would be sold in different denominations to suit different types of posters and would be priced at roughly $1 per credit.
Each post and new thread would cost 1 credit.
In order to allow adequate discussion, once a user has paid for and commented on a thread, future posts in that thread would not cost additional credits.
Just a random idea that occurred to me today and definitely not fully fleshed out...but I thought it could have some potential:
1. Having an entry price point would mean that the users who post are (generally) people who are serious about the topic and there to have discussion.
2. The credit system would help reduce spamming since it costs people to share their thoughts. While empty posts like "OP is wrong and stupid. End." are still bound to appear, they'll hopefully be followed by some semblance of an argument to go along with it.
3. Repeat threads wouldn't be as common. Since it would cost a credit to start a new thread, people may actually spend some time searching for a similar topic rather than wasting their money.
4. Similar to any pay2play game, the threat of getting banned or suspended would help regulate users from doing anything that goes against the User Agreements.