And then anyone who doesn't have extra resources is prevented from using your service... As if it is a trivial amount, spammers will pay it, and if it is non-trivial, then some people won't be able to afford it. Doesn't seem like a great idea.
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.
I was going to propose that it's only returned on good behavior, but that gives sites an incentives to harshly police their new user population.
Ideally you'd have some sort of service that both parties trust that is holding the BTC.