Locate the Activity you would like to delete the Ratings on via Find Learning or Content Manager (see Find Content in Find Learning or Content Manager if you're unsure how to do this).
Click on the Common Tasks button (Find Learning) or its title (Content Manager).
Click on the Ratings tab on the left-hand side.
Click on the rubbish bin icon next to the comment you would like to delete.
Click on Confirm in the popup window.
The Rating will be removed.