Hmmmmm........
If you are still fond of, and respect, this person you might have a face-to-face and a very long talk where you gently, but firmly, explain your position. You did find her/him attractive enough at one time to form a relationship with that person. Give them the courtesy of closure.
On the other hand, you may have done all this already and are now being subjected to a campaign of harrassment. In this case I would consider the following.
***If she/he is leaving endless messages on your answer machine you might leave a temporary message on your service:
"If you are Miss Ex then would you please stop harrassing me with these constant phone calls. If you are someone else that I'd actually like to speak to then please leave your name and number and I'll get back to you later."
***I would also just hang up without engaging in ANY conversation the minute she answered.
***You might also consider being offline on your MSN messenger for a short while???
A friend has recently been unmercifully harrassed by a person wishing to be more than a friend by phine, MSN messenger, email and on her forum!!! It has been an absolute nightmare and has taken a long time but it seems they may have gotten the message...
In her last face-to-face conversation with this person she told him that she did not find him attractive physically, mentally or emotionally.... and that she longer she knew him the more repulsed she was by him!
That evening he left a message on her phone saying that he forgave her for all the mean things she said to him and that he understood that it was probably a hormonal thing.............!!!!
When he rang again the next day, she absolutely screamed at him and hung up. It's been quiet so far... but we (she and her friends) are awaiting further developments. And yes, she has threatened him with police action.
So, it's a world-wide problem and afflicts both sexes.
Good luck!