Go to your c drive (or your drive where windows is installed) and go to program files and in there is a folder called messenger. Just rename the folder to do not use or something like that (mine is messengerdonotuse and that stopped it!). Happy messenger killing :)
OOps forgot to tell you. Remaning the file won't work until you open up task manager and go to processes. Stop the one called msmsgs and then rename the folder.