Prevent Invites From Automatically Appearing On Your Calendar
If you need to stop outsides individuals from adding events to your calendar on their own, there is a setting you can set that forces the invite to come as an email and then only show on your calendar after you accept the invite. To set this:
Go to calendar.google.com
Find the settings gear in the upper right and then select Settings
Scroll down to the Event settings section
Click the drop down for Add invitations to my calendar
Change it to one of the following:
When I respond to the invitation in Email
This makes all invites require your approval via email first. This is the most effective
Only if the sender is known
This will require invites from new users to require approval via email but will allow users who have previously been in your contacts or earlier invites to add events. This may still allow for unintended users to add events to your calendars if you accepted one previously.