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For disabling comments:
1. Dashboard -> Settings -> Discussion, untick Allow people to post comments on new articles. You can notice the paranthesis from that option (These settings may be overridden for individual articles.)
2. Disable comments for each old posts and pages because, as said the parenthesis, the comments is still allowed for those. The easy way, in the list of posts (or pages) choose the little "Quick Edit" link which appear on hover over each post from list. You will see the option to disable comments for that individual post.
3. At Suffusion Options -> Other Graphical Elements -> Post and Page Bylines choose to hide Comments Link for posts and pages.
4. If you use another posts formats than default (like 'Gallery', 'Status', and so on) at Suffusion Options -> Other Graphical Elements -> Post Formats, for each format you use you can also "Hide the comments link", like for bylines.
5. Finally, at Suffusion Options -> Backend -> Comments Settings you can Hide "Comment form closed" on desired pages or posts.