It can get really irritating if an email containing an event that you didn’t ask for, or don’t want to attend, directly gets added to your Google Calendar. This in effect is spamming your calendar.
Google has now brought in an easy way to keep your calendars clean. If you’ve never interacted with someone, their invitations won’t be added automatically to your calendar unless you indicate that you do know them. You’ll still be able to respond to the invitation sent to you in the email and change this behaviour at any time in Settings > General > Event settings > Add invitations to my calendar. Here you can select whether you would like to add invitations from everyone / only if the sender is known / when I respond to the invitation in email.