Have you ever had someone you don't even know tell you that they love you?

This past week a person on Facebook messaged me and declared that they are in love with me. I think to the point of obsession based on how this person was talking to me. This person also said that they'd fallen in love with me months ago when they saw my picture on Facebook. Anyways, I had no idea that this person even existed until they messaged me. I put this person down easy because I'm already in a happy relationship and it would be wrong for me to give this person any hope when there isn't one. That being said, after I explained that I didn't return their intense feelings and that I was already in a relationship with someone else the person in question suddenly became very clingy. I tried to stay friends with this person because I honestly felt bad, but then it got real intense to the point where I was really feeling uncomfortable talking to them. Long story short, I had to end it very quickly. I had to block them on Facebook in every capacity because I no longer felt safe. This person even made another FB account just so they could pretend to be someone else and become friends with me, but I caught on real quick. I'm scared and I don't know what to do. They haven't done anything illegal...yet, but I'm afraid they will. Any advice? Have you ever been through something like this?