It is not a given that this will always be possible. I could imagine some kind of steganographic watermark in videos - diffused over the entire signal so that it cannot be easily cropped out - combined with a check for the same in all recording equipment that blocks the recording or blacks out the area if detected. Could be done "voluntarily" by all large manufacturers for starters, then eventually mandated by law for all equipment sold or imported into the country.

And there's already precedent for this kind of thing: the way copiers block money bills as source.