If you don't want viewers to be able to freely download your work, you may easily disable the option on your video. Go to your video's privacy settings as demonstrated in the GIF below, to switch off the downloading feature.
If you have disabled downloading, viewers will not see the "Download" button on the video page. Your video will no longer be accessible for download when the setting is well-done automatically.
Of course, if you change your mind at any time, you may enable "allow downloads" to allow users to download your material using the same steps.