MetaFilter is a web forum that charges $5 to create an account. Between that and aggressive but professional moderation, spam basically doesn't happen since a couple of obvious spam comments and you're permabanned.

However, it limits the user population to people with a debit card and $5 to spend. A bitcoin-based solution right now would be even more restrictive, since apart from buying bitcoin in person, you pretty much need a bank account to acquire them.