Google has improved its Calendar settings to help you better manage your invites as well as spam. 

You now have the option to add invitations to your Google Calendar only when you’ve responded to an email event invitation.

To enable the new feature from Calendar:

• Click the Settings gear icon.
• Select Settings.
• Scroll down to Event settings.
• In the field Add invitations to my calendar, select:
  • When I respond to the invitation in email.
    • This new RSVP setting will make sure you only see events on your Calendar that you’ve responded to. Enabling this setting will only apply to future events, and you’ll have to manually remove any previous spam invites that you’ve received.
    • You will also receive an email for all event invitations going forward (aside from event updates) even if the organizer has chosen not to send one upon the creation of the event.
      

For more information, visit Prevent unwanted invitations from being added to your calendar.