If this is the Windows Messenger service (as opposed to MSN messenger) popping up with spam messages, a little program called Shoot the Messenger is available at
http://www.grc.com/stm/shootthemessenger.htm which will disable it.
If it's MSN messenger, go to the menu and clcik on Tools then Options. Under the General section there are some checkboxes. Uncheck the ones to automatically run messenger when Windows starts and to allow automatic sign in when connected to the net. After that MSN messenger won't load and sign you in whenever you go on the machine