Attachment posts aren’t exactly the worst thing in the world. They increase the size of your site, and can contain relevant data if you’re using alt, description and title tags. HOWEVER, I do completely see your point, but the good news is that Google won’t crawl those pages if they’re not linked anywhere or included in your XML sitemap.
Here’s a WP Beginner’s tutorial from a few years ago. I’m a little reluctant to send you there because it references kind of old plugins, but the one note they make is to auto-forward all attempts at accessing your attachment posts by putting
<?php wp_redirect(get_permalink($post->post_parent)); ?>
in your first line of image.php for images specifically, or, I think what might be more global would be attachment.php.
Also, I just checked if the Attachment Pages Redirect plugin they reference is still being kept up, and, as of 4 weeks ago, it is, so that’s another alternative.
Otherwise, I’d be on the side of leaving it alone, it’s turned on by default for a reason. Mind you, that could change, but the few articles I’ve read say that there is the chance that keeping them open increases your SEO (but who really knows).
Anyway, good luck!