the trick of the credit card/ one identity per user trick is that you must not only charge; that's just a way to make bans meaningful... you must also ban user

the trick of the credit card/ one identity per user trick is that you must not only charge; that's just a way to make bans meaningful... you must also ban users who are not contributing. The hard part is crafting a policy such that there is a clear way of dealing with earnest cranks (or people willing to be seen as earnest cranks.) But even if you don't solve that problem, you can get rid of the obvious trolls by implementing some 'one identity per human' plus banning. Heck, if you raise the price of identities enough, you could make bans meaningful without limiting each person to only one identity (though, I suspect, in that case you have to charge more than a buck.)