There’s a girl that keeps talking to my fiancé, although he has never talked to her, and she’s falling in love with him and threatening to hurt herself if he blocks her, but she is causing problems between us, and she is sinning through talking to him (since she is talking about forbidden things), and he wants to block her but he’s afraid she will harm herself, so if he does block her will be held accountable for any harm she does for herself? Although he didn’t lead her on or anything?

No he is not accountable it sounds like she is blackmailing him into haraam and he has to save himself from that.