My Messenger was hacked - Need help

Hi,

I'm here for help with MSN Messenger. I have the App installed on my Iphone and within the past two weeks when I've been chatting with my little sister or her friend someone takes over the conversation and starts chatting with me. During the time the hacker is talking to me me sister mentioned that she cannot send me any messages and her screen goes blank. I have changed my password a few times but it keeps happening. Is there a way I can find out who is doing this? They appear to know who I am and are also trying to tell me that they are my ex but I doubt he'd care enough to do this. Still, I am curious that it may be him. Please tell me what I need to do to secure my system and maybe find out who is doing this to me. Some of the messages are really creepy....

eg - "PAULDOESN'TWANTTOHEARYOURBREATHING"

Thanks!
Jaime

 

Thank you for your suggestion, but I tried that and it didn't work. I took the app off my phone and then wiped my phone and changed my password from the computer. I then did a scan on my pc too. A few hours later I tried logging in to Messenger on my pc and was chatting with my sister and then noticed that the hacker was chatting with me pretending to be her again. This time I was receiving a message from her with her user name and then another than was coming from the same account but a different thread and it just showed her e-mail address. I called her after and she said that she was not online. An IT friend told me that it's her computer that is probably compromised so I told her to change her password again immediately. do you think they have access to my system? I cant tell. Thanks for any advice!