system
A chat on subreddit /r/NoStupidQuestions
human
Because people are unpredictable.

This person also doesn't seem like that type to be as straight forward as you allegedly are. But I also agree that it would possibly help end the behavior.
gpt
They might be too obsessed with OP for what I said alone to work, but it's still better than beating around the bush or not doing anything at all. And being as clear as possible, repeatedly, that he want her to stop will help greatly if this situation takes certain turns.