Outlook – how to stop sharing a calendar in Outlook 2010

I once shared a calendar on icalx.com from Outlook 2010.

I do not want to share it anymore and I get constant pop-ups from Outlook requesting me to provide a login / password to access icalx.com (which is very annoying).

How can I "unshare" a calendar so that Outlooks stops prompting me?

(I looked through everything I could find by right-clicking on the calendar without success)

Thanks!