Crowdsourcing the spam identification would be a great move.
Unfortunately it would also reveal to customers that there are fake reviews, causing them to lose confidence in and lessen the value of reviews. They may then read reviews and subconsciously be evaluating if the comment is fake, not if the product is a good buy.
If you hid the "Irrelevant Review" button under a dropdown it might work better, since it wouldn't be there as a constant reminder.
Unfortunately it would also reveal to customers that there are fake reviews, causing them to lose confidence in and lessen the value of reviews. They may then read reviews and subconsciously be evaluating if the comment is fake, not if the product is a good buy.
If you hid the "Irrelevant Review" button under a dropdown it might work better, since it wouldn't be there as a constant reminder.