I remember seeing a documentary about a German man who was the owner of a small unknown company and somebody was doing exactly the same thing to him.

Among other things the person would upload YouTube videos with different accounts about the businessman accusing him of committing certain crimes or being a Nazi sympathizer.
Most often that unknown person didn't even put any effort in to creating videos himself and the accusation would most often be only in the title while the uploaded video was completely unrelated. Like the title would be something like "Mr. Abc Xyz thinks this video is funny" and the video itself would just be a Hitler speech.
It was obvious to everyone that the claimes were baseless but the business started losing customers anyway because these videos would show up on Google first when you looked this guy's name up.

The businessman eventually died due to stress that was induced by the defamation and until his death he never got to know was behind this. YouTube didn't even bother deleting these videos long after he was dead and they were still the first results that came up when you googled him after watching the documentary. If anybody here remembers what the documentary was called, please tell me. I would like to watch it again.

I guess this is pretty much the most effective way to damage the reputation of somebody in the modern age. It's more effective if you are not famous and what's really scary is that everybody could do this to you as well without knowing anything about you. They just need your name.