If you're truly concerned about users clicking so fast that they can bypass the partial second it takes for disabling NEXT, then don't use the built-in player navigation. Instead, use custom NEXT buttons on each slide with an initial state hidden.
Yeah, that's a lot of extra work. Is it worth it? Do you have evidence that people are clicking fast enough to bypass restricted navigation?
The ideal solution is to convince users it's in their best interest to complete the course, so they won't try to click through without viewing/hearing all the content.
Alternately, see if you can pull time-spent amounts from the LMS, and then tell people that anyone who finishes too fast will be punished.