Spammers have found a way to add unwanted phishing and scam events to people's Google Calendars without their permission. To prevent spammers from adding things to your calendar without your permission.
As explained in OneZero by How-To Geek's former editor-in-chief Whitson Gordon, Google Calendar (by default) displays invitations to events, if the user so wishes, or not. Spammers use this gap to flood the calendars of people with malicious links and other derogatory text.
Dismiss calendar from automatically adding invitations
One of Google Calendar's default settings is the ability to automatically add calendar invitations to appointments. To turn it off, click the gear icon in the upper right corner of the display and then choose Settings.
Next, select "Event Settings" "Found in the left sidebar.
 Find the drop-down menu labeled "Automatically add invitations." Click the arrow.
Select last "No, only show invitations,
Preventing calendars from adding events from Gmail
Google Calendar also retrieves events directly from Gmail If an invitation is sent by email – sometimes even if it is moved to the spam folder – it will be saved to your calendar – to disable this feature n, return to the Settings menu by clicking the gear icon and selecting Settings.
Then select "Events from Gmail" in the left sidebar.
Uncheck the box next to "Automatically add events from Gmail to my calendar."
Google Calendar asks you to confirm this action. Click "OK" to confirm the change.
Hide Rejected Calendar Invitations
If you've gone through the spam calendar events and declined, they'll still appear in Google Calendar, but it's crossed out. The reason for this is that, by default, declined events still appear.
To disable the feature, return to the Google Calendar Settings menu. As previously mentioned, you can get there by clicking the gear icon and then selecting Settings.
Choose "View Options" in the left sidebar.
Uncheck the box next to "Show Rejected Events" to remove the reduced invitations.
Hide Rejected Events on Mobile Devices
Oddly enough, if you wish to hide rejected events in the desktop view, they will still be included in the Android, iPhone, and iPad Apps displayed. If you do not want them to appear, launch the mobile app and open the overflow menu by tapping the Hamburger icon in the upper left corner, scrolling down, and tapping Settings.
. Then select "General".
. Finally, turn off "Show Rejected Events".
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A growing number of online users has reported that this tactic has been used to send spam to their Google calendars. Google has not published a response to the incidents at the time of writing. However, it is hoped that some of these defaults will be disabled in the future.