Regretting getting back in touch with an old friend

Hi. A few years ago I met a girl through some friends and we got on well. She was smart and interesting and we started hanging out. However it soon became clear that she was extremely needy and would expect me to talk to her every day. If I didn't call her she'd get very upset and she refused to listen to any of my problems. Last year I was going through a lot of stress in my life and I cut contact with her because she was being incredibly demanding and stressful. For months she e-mailed and text me with various threats and emotional blackmail (for example, "I'll wait outside your father's work and beg him to speak to you" and messaging my brother's friends on Facebook asking them to urge him to get me to speak to her). I finally replied and apologised for disappearing and said I'd stay in touch but in a less intense way but I'm regretting it already. I thought it would calm her down but it hasn't. She keeps going between "How can I make things be like they used to be?" and "You should respect the fact that I am angry with you, this won't work". I just want to be rid of her once and for all but I don't want to hurt her and I don't want her contacting my family. I don't know how to sort this like AT ALL apart from going back in time and never speaking to her. Help?