Blaming,disturbing and defaming

Hello sir/madams.this is a very awkward case which is disturbing me a lot and hampering my studies.I am prepairing for civil services. I had a friend..as now i do not consider him friend anymore.he is blaming me that i have an affair with her gf. At the beginning..i thought it was just a normal doubt and i helped him in clearing things as much as possible. But he started to act like a psychic like coming to my home at 6am couple of times and calling me many times a day. Keep asking me is there anything between you and her ? Calling my sister many times.asking me about every second..where were you,why were you there ? This may be appearing simple but this situation gain extreme levels in a month and i feel being tortured.he is blaming to common friends.. but even they know me and are saying.. i can never do such a thing. But still his mental illness or i dont know what is up in his mind.Even he is trying to threaten me by contacting some foul person.i does not fear of him.but i am a law abiding citizen and i do not want to do anything against the law. And i even dont let this thing loose, As i believe he has gone mentally ill and he could take law in hand. So i think its better to turn down such actions before they took place. Thankyou. I'll be very grateful for your reasonable helpful replies.