Making people pay for the privilege of posting online isn't a good idea - getting people to use a forum for free is hard enough, just try getting Internet people to fork over "the price of a cup of coffee" as well.

If you want decentralized spam resistance at scale, Web of Trust is the only solution that works. To register, I contact a node that adds you for free by filling out a captcha, talking to them on IRC, etc. They add me at a trust level just above zero. If I start off by posting spam, my account instantly dies.

So I have to first post a bunch of productive comments that people respond to, and then I can start spamming.