Getting harrassed online

Ok so this guy on Yahoo messenger contacted me. He was nice to start off with. Then he started calling me nutter and a mental case without me doing or saying anything much at all. He said I needed help and I keep blocking him but somehow he keeps getting through. All I said when he asked did I like sex was that I had no interest in it. He then said I had mental health issues and that I was off kilter and said I needed mental help. He threatened to find out where I live and stalk me too. It made me feel upset and really sad. What can I do?