I got the option to cancel auto start when I first launched it. Win 8.1x64.
Does right clicking on the icon in the taskbar give you any options? NOPE, tried it.
But you can always use the old fashion way and delete the shortcut from the start up folder. It is a hidden folder so you have to unhide, et, etc. When I had Win 7 I actually kept a shortcut to the start up folder avaulable for this very reason.
HTH