Comments and ratings are a great way to interact with clients on a private gallery.
To prevent Spam comments if you allow commenting on a public gallery, it is highly recommended you do not allow anonymous commenting, and preferably you require commenters to have an account.
To delete comments on your website, log onto your website with the same credentials you use to access your PhotoDeck admin space. Open the page with the comment to remove: you will notice a Delete button next to the comments.
Ratings are useful to help a client make his selection of images, as there are shortcuts to add 5-star images to a lightbox, to download them, etc...
The last rating on an image or video overrides the previous one, so it is usually not a good idea to allow ratings on public galleries.