You would have to sign each file individually after checking it of course for this to work.
The sources and their comments (signed or not) are gone when the file is completed and you can check it. You would have to store a list of all signed file names for a download to judge later who was good/bad. Complicated.
IIRC public/private key feature is already used in user hash verification and could be reused for something like that (didn't check though).
The "verified" flag could be transported through the comment tag without extending the protocol (which is not ours to change). So you could simply blacklist user hashes who verify something that turns out to be faked. Be careful what you verify though. What if someone accidentally verifies something which still is fake afterwards, like a movie that starts fine and then is filled with crap? You could end up with regarding other files shared and verified by him as fake although they are fine.
A good hard look at the list of shared file names has served well for me to unveil most fakes without such a feature.
