Hi
Did you come right with removing the calender yet?
You mention the calender is showing under My Calenders, I Suggest you would have to remove/close the mailbox associated with it and the calender will automatically disappear.
In outlook 2010 you go to:
File, Account Settings, Account settings again, Under email account, select the Email account, Click Change, on the Server Settings page click more settings,
Navigate to the Advance tab, and under Mailboxes, there should be a mailbox there that is related to the calender,
Select the mailbox and remove it, Calender should be missing now as well.
The Delete Calender Option is greyed out because the full mailbox has been added in outlook,
The only time you can delete calender are when they have delegated it or shared group calender.