Go to "Settings" and choose "Passwords & Accounts":

Click on "Subscribed calendars":

Then click on the Smartplan calendar:

Delete the calendar by clicking on "Delete account":

Find the Smartplan calendar in your list to the lift and 'right'-click on it. Now you can click "Unsubscribe"

Hereafter you just click 'Delete' and then it's gone

Find your Smartplan calendar in the list to the left and then simply click on it and choose "Remove":
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Place the mouse on your Smartplan calendar in the list to the left and click on the cross which will appear:

You will now be asked if you are sure you want to delete the calendar. Simply click on "Remove calendar":

The calendar has now been deleted from your Google Calendar.