From MatrixWorks: "Any user must be registered and logged into the site for views or downloads to be payable"
I have to admit I still oppose this idea strongly, because I don' think it's going to accomplish the desired goal. The scammers can sign up for all sorts of bogus accounts quite easily, all they need are seperate email addressess, and it's not realy difficult to setup a few hundred on yahoo, hotmail or any of another dozen places out there with free email accounts.
So requiring a login for download in and of itself doesn't really stop the scammers (though admittedly it does make there job a tad bit more difficult), all it really does is make the board far less friendly to people wandering in off the street. I think the damage you do with that perception far outweighs any good that comes from requiring registration for download.
I think most of the rest of the ideas presented have merit, but the pay per download idea I think is not really going to have the desired effect, indeed I think over time it's going to cost the site a lot in lost revenue.