If you had shared the iCal with others and no longer want them to see the calendar feed, you can revoke their access:
Go to Properties
Click on a property name
Look for the Export calendar below the listing(s), click on the three dots beside the Copy iCal link button.
Click Revoke calendar access
Our system will generate a new URL, and invalidates the previous URL generated which means that everybody who was using that feed will no longer have access.