I don't think you could use RLHF to stop plagerism. RLHF can be used to teach what "angry response" is because you look at the text itself for qualities. A plagerized text doesn't have any special qualities aside from "existing already", which you can only determine by looking at the world.

One thing you might do is use a full-text search database of the entire training data. If part of ChatGPT response is directly copied, give it the assignment of "please paraphrase this" and substitute the paraphrase into the response. This might slow ChatGPT down a lot - but it might not, I think an LLM is actually more computationally expensive than a full-text search by a lot.