As far as I am aware, the only way to do this with absolute certainty would be to build a totally new media plugin that rendered the video within canvas.

Otherwise, all you can do is put more steps in the way of the user being able to skip. For example, you could hide the 'seek bar' but then most browsers have in-built controls that could be used to bypass this. So in IE11 I could right-click and set the playback speed to '8x' for example.