For editors: usage

Open the File list module and edit the metadata of a file (or open the file resource in the Media module).
Switch to the SEO tab, enable Do not index in search engines and save.

The "Do not index in search engines" checkbox on the SEO tab — The checkbox on the SEO tab of the file metadata form.

That's it. The file's robots.txt entries appear immediately — no cache flush needed:

Generated robots.txt with disallow entries — The generated `robots.txt` with the disallow entries for the marked file and its processed variants.

Unticking the box removes the entries just as immediately.

Important

Already indexed images disappear only after the next crawl (days to weeks). For immediate removal, additionally use Google Search Console > Removals.