Girl threatening to file false case

I was in contact with a girl through social networking site before 3 years. But We don't chat . After 1 year we satrted chatting online with her. Initially she was flirting with me and I also . I understood that she is so mature and she will take the relationship as close friendship . After 6 months of chatting she proposed me to marry . I clearly told her I am married and I have a child also . And don't want to marry further. I did not disclose that I am happy with my family and I am with them as I don't want to hurt her emotions. Several times I blocked her for 2 months . But later she again got touched with me. I thought she is just like to chat with me .so I continued it for 1 year . But now I met her for the first time and had little gossip on road . After that she got to know that I am staying with my family . Then she started threatening me and blackmailing me that she will file false case against me that I have physical relationship with her and cheated her . But in real I don't have because we just met once each other that to be on road . Kindly suggest what to do and how to escape against the false fir if she launchs.