How to cope with someone in constant, obtrusive pursuit of your significant other?

Have had a tumultuous but rewarding relationship with my boyfriend of two years.

His ex girlfriend, after all this time, is still in constant and frightening pursuit of him, although not enough to warrant a restraining order. A few days ago, I learned that she had a Japanese character representing him tattooed to her left ring finger, symbolizing her commitment. She writes him excessively long emails begging him to come back to her. I approached her about it via email as neither my boyfriend nor myself have face to face contact her, asking her to stop. I didn't lose it on her, it was concise and respectful. Her response, predictably, was along the lines of "fuck you, I do what I want".

I struggle with this immensely. I feel powerless and it's unnerving to know this woman is actively trying to steal my boyfriend. I'm pretty secure so I had not obsessed over it recently until I learned she has just had the tattoo done. Any advice on coping?

EDIT: I am 21 female, he is 24 male.