How can i find who is playing pranks with me?

I am from Mauritius, i am getting anonymous sms since long from a number. Now the person is writing anonymous letter to me. This is adding salt to my wounds since i am divorced with my ex. The person causing harm to me is not leaving any evidence for me to find out who is doing so. I am convinced that its not my ex. So can any one give me any hint how to find that person. I do have her phone num, but now the sim is blocked and the authority is not able to find out the person