Here's what I did: I clicked on the the calendar. The ICal tool bar comes up. Under iCal, I clicked on Prefereneces. A box came up; I clicked the Accounts tab. In my case the "Enable this account" box was checked. I unchecked it, and the problem went away.
I followed bobztrainz suggestion. I have no clue why a box in iCal (which, incidentally, I have never used) should have anything to do with access to the iTunes Store; but it did indeed work. All of a sudden I had no access to the iTunes Store, but this worked for me. Thank you