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There isn't. It could serve as a warning sign (where new entrants can think before entering) instead of a actual report.

However that is still prone to abuse as people can flag a legitimate one to reduce competition.

I don't think that would work for a couple of reasons. What happens if a user comes across a suspicious giveaway, one which they can't guarantee is fake? Do they report it? If so, they'll lose their opportunity to enter. They shouldn't have to make that decision, and you'll simply discourage people to vote overall. I think we'll have to test it out, and see what giveaways rise to the top of the reported list.