I don't think I ever have? I think back in 2011 or 2012 or something there was an (actually very sweet!) review on one of my stories, which inadvertently gave away a big twist at the end of the story. I contacted the reviewer and thanked her, but asked if she would mind removing the spoiler. She was very cool about it, but it turned out she could not remove or edit her review. I also could not remove her review, because it was not anonymous.

I contacted one of the mods and (politely!) said that both the reviewer and I would like to have the review removed, but I never heard back. Some weeks later, I tried writing the mods again, and again, nothing. The review with the spoiler is there to this day...