The messages exploit the default settings in Google Calendar and fortunately this is easy to fix.
Open up your Google Calendar, click on the Setting icon in the top right of the panel (the Gear wheel icon) and then select 'Settings' from the menu that appears.
That gives you a long page of settings - look for this one, most of the way down:
Change the current setting to 'No, only show invitations to which I have responded' and Save your settings.
That should do it !
Why Google doesn't have this as the default I have no idea...