You need to understand that in order for the human user to view the content, the browser (or some other software) need to download it. And when the content has been downloaded to the user’s computer, you no longer control the access to it.
So-called “streaming” is just a download-mode where the standard viewer is programmed to automatically delete the content after it has been viewed by the human. However, this can be circumvented by either using a non-standard viewer (i.e. one that skips the deletion step), or by having another piece of software running that grabs the content from the frame buffer when it being viewed – and saves it permanently.
If it can be viewed, it can be captured.