Magento product reviews are often littered with spam comments, filling up the database with lots of pending reviews. One quick way to combat this is to only allow registered users to post reviews, disabling it for guests.
Allow guests to write reviews – No
Within your Magento Admin find;
System -> Configuration -> (Catalogue) Catalogue -> Product Reviews -> Allow guests to write reviews -> No
Persistent spammers could still register to add a review, but it should greatly reduce the number of pending spam reviews in your Magento store.
Magento Review Spam
This simple config change greatly reduced the amount of spam received and was more effective than an anti spam plugin that the client had previously used. Let me know your experiences in the comments.
I spent a lot of time to locate something such as this