Spammers are able to send spam calendar entries to your Google Calendar if you have it set to automatically add them. Here’s how to disable this feature.

1. While you are in Google Calendar, click Settings in the upper-right corner.

2. Select the General tab.

3. In the Automatically add invitations to my calendar section.

4. Select No, only show invitations to which I have responded.

5. Click on Save.

Source: Spammers Are Taking Over Google Calendar, Noooo