NOTE: Performing these steps from a mobile device can have unintended results. Using a desktop is recommended.
From a desktop computer, log in to the main "Google Calendar" page (calendar.google.com).
On the left-hand side of the screen, locate all of your subscribed calendars.
NOTE: Subscribed calendars will have a checkmark to the immediate left of the calendar name.
Hover over the calendar name you wish to manage.
To the right of the calendar from which you wish to unsubscribe, click the X.
Click Remove Calendar to confirm.