Just as an exercise in 20/20 hindsight, it seems to me that the easiest change to make is to not trust every player implicitly. In other words, you enforce some level of duplication in the work. If enough people agree that this piece goes there, then it goes there, and if not, then it doesn't. A single attacker is unlikely to be able to undo the work of dozens of supporters, even if it takes less time and effort to undermine the truth.

I believe Foursquare did something like this with regards to their recommendations. They'd ask questions about locations (e.g., pick a beautiful picture, is this place classy?, is it loud?, etc), and only when enough people agreed would they use the information in recommendations.