Have had a lot of emails recently about missing reviews. If you get a notification that a new review has come in and then it isn't there when you check, don't panic. It was a spam review and has been deleted.
In an effort to keep spam off the site, I try to delete the spam registrations and reviews as they come in. It is an ongoing battle and there is no real practical way to stop it except to disallow reviews completely.
You can set your stories to members only, but that will only slow it down, not actually stop it.
Every day i delete anywhere between 50 to 5000 or more reviews that are just rubbish trying to sell things.