Shouldn't even be an argument, about the tag situation. Some books have slow-burn moments, where certain tags take 30+ chapters to even show up or establish. Some people put an Adult tag for stuff that shows up after more than 70+ chapters (because they are required to do so). So let's keep it real. Call the idiot out, make it a public comment to inform readers what is really going on, then proceed forward.
As for the title topic of your post however, toxic people exist everywhere; it is after all "the internet." If specific users unfairly review bomb and break their own terms of service in doing so, you report them with proof, and nothing is ever done about those specific users, then and only then would I begin to worry or question the status of how things are in any platform. I don't expect any problems though. In the event the reviewer is not breaking any rules, just block them.
Is this allowed? This person goes around and gives 1 rating.
Whether it is allowed or isn't, I think we can make use of the behavior being displayed. Anyone here who has a book and doesn't want to be included in this user's rating should seek the user and preemptively block them. I am not against outing people who do this.