What should I do with someone who blackmailed me then refused my repentance and threatened to expose me, even though I warned him about God?

The questioner is advised to beware of Satan and his steps. As for the person threatening to expose the matter, it is first recommended to supplicate by saying: "O Allah, suffice me against him however You wish," and "O Allah, I place You against their throats and seek refuge in You from their evils." One can also threaten him by informing the guardian or the responsible authorities. If the matter is exposed, it can be denied using rhetorical ambiguity (tawriya) in speech.