[wiredfool]

The effectiveness of reviews is predicated on the population of 'helpful' reviewers being greater than the population of spam reviewers after some level of filtering.

It's unclear if a crowdsourced flag would tip the balance of helpful vs spam in any useful way for categories where there's already a unhelpful level of spam.

[Super_Jambo]

You could have a reputation system where vote weight is gained for correct identification of upvoted bogus reviews. Then your spammers are forced to fight each other in order to upvote their own spam. Start everyones voting weight at zero until they've spent $x.