You can insert a Google Form in the particular page within the Google Sites where you want users to Submit Comment. That way, all comments will be submitted via the Google Form in that page without being able to edit anything else on the Google Sites. And if you want to display all the comments for users to view, then you insert the Spreadsheet that contains the responses from that Google Form in the page as well.
Please see this example : https://sites.google.com/site/eros84via/home
Other alternatives : http://sitesguide.pjrprojects.co.uk/google-sites/how-tos/add-content-using-a-form