Accessibility

I realise this answer is old now, but you can refine it a bit. For example, if you only wanted to prevent right clicks on <video> elements.

This goes in the same Site administration > Appearance > Additional HTML. > Within HEAD as the previous solution.

<script>
window.addEventListener('DOMContentLoaded', () => {
   document.querySelectorAll(`video`).forEach( ele => ele.addEventListener('contextmenu', (e) => e.preventDefault() ) );
});
</script>

This code waits for the the web page structure ( The DOM ) to load, then grabs all <video> elements on the page and adds an event listener for them preventing the context menu from appearing, but only on that element. The context menu continues to appear elsewhere, enabling useful stuff like copy paste and context menu spell checkers to continue working everywhere else.

The querySelectorAll() method is super useful here because you can make some very complex selections. Youtube embeds for example, are in an <iframe> element, but so is H5P and I want the context menu there.  However you can grab just Youtube iFrames with a wild card selector on the <iframes> src attribute.

<script>
window.addEventListener('DOMContentLoaded', () => {
   document.querySelectorAll(`video, iframe[src*="https://www.youtube.com/embed/"]`).forEach( ele => ele.addEventListener('contextmenu', evt => evt.preventDefault() ) );
});
</script>

Youtube actually overrides the context menu with its own, so good luck blocking that from within Moodle; best solution I had was to put an invisible <div> over it and block right clicking on that element instead.