Open Google Calendar by going to Calendar.Google.com. Tap the settings cog on the top-right of the page and select Settings. Choose "Event Settings." Change the option that says "Automatically add invitations" from "Yes" to "No, only show invitations to which I have responded." Now scroll down the page to the section titled "View Options" and uncheck the box that says "Show declined events." This prevents Google Calendar from showing spam that you've declined.