The real answer is that if the learner cannot download the video, they cannot view it. If they can view it, they have already downloaded it. Youtube spends a great deal of time trying to keep people from downloading videos, and LOTS of people spend a lot of time finding ways to work around the restrictions. If you were to host the video on a place like Youtube or Vimeo, you might be able to make it more difficult for someone to download it, but trying for more than that might not be worth the effort.
In my opinion, your effort is better spent making content that people will want to download and see. Once you reach the point where millions of people want your content, then maybe that would be the time to worry about piracy.
