Removing account from iCal to which I have lost access
I tried Kvass's solution, it wasn't easy to click the "accounts" in the split second after closing the error window, but I did it. 

I then disabled each of my ical accounts and the error window went away.

Then I went back through activating each account one by one until the error message appeared again, narrowing down which account was the culprit. 

I then disabled that account permanently and re-added the calendar events from that account to my good one. So far it's been 20 mins and it's working. 

Not the most time efficient or technical solution, but I finally have relief!