Magento don’t provide admin screen where store owner can delete all the reviews and ratings of the products. The easiest way to delete them is using raw SQL queries, which you can run in Mysql console or in PhpMyAdmin. Remove/delete all the reviews and ratings of products in Magento using simple SQL queries given below.

truncate table `rating_option_vote`;  
truncate table `rating_option_vote_aggregated`;  
truncate table `review`;  
truncate table `review_detail`;  
truncate table `review_entity_summary`;  
truncate table `review_store`;