Delete Sync Data
Log into the Zulu admin page
In the top right corner, click the link "Delete Sync Data"
Now check the "Calendars" checkbox, and Select all Calendars you want to be deleted.
WARNING!!!: This displays all calendars corresponding to Google Accounts, and it will permanently delete these calendars from google. Only check the ones you want deleted.
Click delete, and your sync data will be deleted. It will be rebuilt on the next sync.