How do I close an outlook.com address that has been set up in my name

Someone has hacked into my btinternet email account and has set a new outlook.com account in my name. I am trying to close it but because I do not know the password that have been set up Microsoft tell me it is not my account so I cannot close it. So I am stuck with getting lots of undelivered mail being returned to my BT a/c also I have had over 800 addresses added to my contacts list. The hacker sends out appealing letters in my name and has hacked into other email accounts as well but is using my address.