General help

You can't.

It's nothing to do with Moodle it's the way the internet works. If the data is sent to your PC then you can download it - usually very easily.

I will be as blunt as I usually am about this - if your business model relies on your students not being able to "steal" your videos then it's not a very good business model.

I think I found a way to do this.
I store my videos on box.com. The service has a feature to put the video within iframe tags to embed the video elsewhere. There is an option to make the video "view only."
This will not make the video absolutely secure from downloads, but at least there won't be a working download button.

I am not sure exactly how this works, but here is a sample:

<iframe src="https://app.box.com/embed/s/6i7fofilvtkcy9vvhqg9i6kpv6y6dlwl?sortColumn=date&view=list" width="800" height="550" frameborder="0" allowfullscreen webkitallowfullscreen msallowfullscreen></iframe>

For this to work, I cannot be logged in to Moodle as admin, and I cannot be logged in to box.com at all. Even closing a tab is not enough, I have to log out of box.com.

Hi,
I use vimeo to host such videos.
In vimeo paid plans you can set the Domain-level privacy so you will able to host a video only on your site and there will be no easy way to download it.
But as mentioned by @Howard Miller - if someone will try hard enough he will be able to do it (the most easy way will be to screen record it).

for most users you can just hide download button , disable right click ...
this prevent users from easy and ready availble download button
but for complete protection : no way , even if you find a video provider that do it m also who want to download will find a way

I've been struggling for quite some time for ways to prevent videos from being downloaded, and found an extremely easy way to do it (of course, this will not completely prevent the videos from being downloaded, but will help a lot, as most users will not find a way to overcome it).

Just add this after the opening of your video tag: oncontextmenu="return false;" 

When you set oncontextmenu="return false;", the right-click menu will be disabled for your video (while it will still be working for the rest of your site). As simple as that, and no one will be able to right-click and save your video file.

I've never posted to the Moodle forums before, but I thought it could be interesting to share this solution, as I spent a lot of time looking for something and this is a solution that takes 5 seconds to be implemented, doesn't require any changes on the pages' programming, and costs nothing.

Best regards,
Lucas.