How to unsubscribe on Outlook 365

Less than a minute

Open Office.com on your computer and sign in
Click the purple Calendar icon which will redirect you to Outlook.com and sign in
Select the menu icon in the top left corner (white dots)
Select Calendar
On the left hand side, find the menu titled Your Calendars
Locate the ECAL calendar subscription.
Right click the name (Most likely the publisher name)
Select Delete