Go to Settings -> Discussion and untick "Allow people to post comments on new articles". Then uncheck "Allow comments" in the Discussion pane on each Edit page for each published Page or Post. If you do not see this pane, click Screen Options at the very top of the page and check "Discussion".
On a good theme, this should remove all references to comments from your Posts & Pages. However, it's not possible to guarantee this with your theme. In fact I would strongly recommend that you read this article and consider changing to a theme from a reputable source asap.