Reviews that contain spoilers will be hidden publicly on the NetGalley site, because spoilers in reviews can ruin an intended surprise for fellow readers and can diminish their reading experience. Once a review is flagged by the publisher or the NetGalley team as containing a spoiler, that review will no longer appear publicly.
For reviews appearing publicly on other websites (such as Amazon, Goodreads, your blog, etc), we would recommend that you at least add a "spoiler alert" warning to your review.
Please keep this in mind for reviews you submit via NetGalley. You may find this article helpful, as it contains tips for writing reviews.