Delegated Administrator does not remove calendars. Use the csclean utility to remove all calendars for any users marked as deleted with Delegated Administrator.
Use csclean to remove all calendars for users marked as deleted but not yet purged.
For example, to remove all the calendars for users marked as deleted in the sesta.com domain in the last 10 days, the command would be as follows:
csclean -g 10 clean sesta.com
If the user has already been purged from the LDAP, then you must use cscal.
For instructions, see To Remove All Calendars of a User Deleted with csuser in Calendar Server Version 6.3.