You can't. Simple as that Even if you did disable it with javascript all the user would have to do is disable javascript and hit printscreen.

Agreed.

It might actually be easier to disable the internet, than it is to actually prevent someone from taking what they want from it.

Even if you could disable every single method for every single browser, it is easy to run video out and record it.

The only way to protect content from being taken is to not put it on the internet.