Just curious--why do you want to hide the link?
Also, not to ruin your day, but do a CTRL-N when your presentation opens. See how the presentation opens in a new window with full browser controls and the URL bar. So you probably want to do hide that also.
I don't recall this being brought up before so you may need to look elsewhere for possible answers.
A Google search turned up this on disabling right-clicking in general on a page:
Disabling right-click - JavaScript code
And preventing CTRL-N:
AS Workshop » Disable ctrl + n and other ctrl + key combinations in JavaScript
I'd get someone who knows JavaScript well and have them implement any solutions you find for you. (That's experience talking from one who is not a programmer but who has fiddled with JS and messed some things up royally--good think I kept backups.)
BTW, are all your learners on one browser or do they use various browsers and versions? Always simpler when you have just one browser to program around.
Best!
- Gerry