The quickest way to turn off commenting in the carousel or attachment pages is to turn off commenting for your entire site in Settings>Discussion and then enable commenting per post/page. You can enable it in the Discussion module for each post/page. This will work for future posts, but may not work for posts you've already published.
For already published images, you can disable the image comment box by editing each image from the Media Library. Open the image for editing, head down to the Discussion Module and uncheck the appropriate boxes and then update your Media. Unfortunately, you will have to edit each picture individually, as there is no "bulk edit" feature for Media like there is for Posts/Pages (other than to delete them).
To the best of my knowledge, reblogging only works on Posts. Have you turned off the Reblog button on your site? If not, you can do that under Dashboard>Settings>Sharing. Once you disable the reblog button, it seems that even if the button appears, it doesn't actually activate a reblog if the button is clicked. I've tested this in general on posts, but haven't yet tried reblogging only an image from the carousel. If you'd permit me to test this, I'd be happy to do so. Just let me know. :)