J MAZ,
First select the calendar that you want to delete in the CALENDARS sources pane, then choose iCal>File>Export… and save the file to your Desktop.
Next delete the the calendar from the sources pane using iCal>Edit>Delete.
Now select iCal>File>Import…, choose "Import an iCal file" (Import), and you will be presented with an "iCal: Import" window. Select the exported file with (Import) and you will be presented with an "Add events" window where you can choose which calendar to use for the imported data.
;~)
