"Calendar Pinning" allows you to "pin" a friend's calendar to your list of calendars. You can think of this calendar pin like a bookmark or shortcut. Your friend's calendar is not in your account, but the calendar pin allows you to quickly access important calendars shared with you from your own list of calendars in the Navigator.
When you are looking at your list of calendars you can see what calendars are yours (they have a calendar icon beside them) and what calendars have been pinned (they have a pin icon beside them).
To unpin a calendar from the list of calendars in your account all you need to do is click on the pin icon next to the calendar. Once you click on the icon you will see a pop-up dialog that asks if you want to remove the pin. Follow the steps in the image below:
Team Account Calendar Pins
If you are part of a Team Account then anything that is pinned in the master account for your Team will appear in your account's list of calendars. You do have the ability to unpin a calendar that has been pinned in the master account for your Team from your account.
You can tell if a calendar has been pinned from the master account if you click on the pin icon next to the calendar in question and you will see the below message: