How to unsubscribe external calendar from iCal

I recently started italki and there was this 'Get iCalendar Feed' function that automatically syncs with my iCal when I subscribe the feed. I did not know exactly what this does at that moment.

Ever since then, the 'italki app' appears in my iCal and this is SUPER annoying. I tried deleting it and it disappears just until I book a new lesson. As soon as a new lesson is booked, the calendar comes back. No matter how much I delete it. I talked to italki and they are helpless as they said they do not know anything about iCal as it's Apple product. Please please please help me delete this italki app PERMANENTLY.