Perhaps you could have a system where users vouch for other users who make good submissions, and then have submissions ranked with something along the lines of PageRank, with "vouching for" taking the place of "linking to".
The advantage of identifying good users over identifying bad ones is that recognition of goodness can propagate: if you believe A to be a good user and A believes B to be a good user, that is some indication to you that B is a good user. Replace "good" with "bad" and this no longer holds.
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Perhaps you could have a system where users vouch for other users who make good submissions, and then have submissions ranked with something along the lines of PageRank, with "vouching for" taking the place of "linking to".
The advantage of identifying good users over identifying bad ones is that recognition of goodness can propagate: if you believe A to be a good user and A believes B to be a good user, that is some indication to you that B is a good user. Replace "good" with "bad" and this no longer holds.