Based on the this thread here and my today's experience, I would like to make a suggestion that I feel all would welcome (except scammers and cheats, of course).
We know that Amazon welcomes product reviews from all the buyers, verified and unverified (i.e. those who purchased the said item from Amazon or not). And that's okay. We cannot contest or say anything in that regards as we may not know if the reviewer is lying or not as that item is available to be purchased at retail stores.
But, with Merch by Amazon, it is clear that all Merch by Amazon T-Shirts are sold ONLY via Amazon.com. Therefore if an unverified reviewer is leaving a bad review, it should be very clear that he/she has an extremely low mentality and is trying to game the system by making his peer's listing look bad by leaving bad reviews.
Such people should be already banned, but that's a talk for another day. But for now, what I'd like to suggest is - what if we stop/disable unverified buyers (people who has not purchased) Merch By Amazon t-shirts to leave a review and get an error that they have to be a verified buyer to leave the review? That'd stop these bad people to a greater extent. I know for the fact that Amazon does this on some of their very own products because they know they are not available for sale elsewhere.
Why not do same for Merch?
Any constructive feedback on this idea is welcome by Amazon staff and other MBA members.