Is there a way to prevent visitors from downloading/ saving content from my website (images, audio, video exc.)?
And if there is, how efficient is it? Would someone with more than average knowledge be able to surpass it?
I would say that there is no easy way to do this. To be able to view your site people have to be able to download the material to their computers. And once it's there it's not too difficult to save it.
But there are technologies that can "watermark" images, for example, to make it easier to trace misuse.