If you try to delete a calendar using Apple OS X Calendar 6 or 7, it will likely keep reappearing. This is because Calendar tries to copy over all tasks from that calendar to a new calendar collection with the same name.
Since SOGo supports events and tasks in the same collection, Calendar re-creates a calendar collection exclusively for tasks contained in the collection you are trying to delete.
To avoid this issue, visit the SOGo’s web interface and open the properties window of the calendar you want to delete. From the properties window, uncheck the Show tasks checkbox. Tasks will no longer be sent over CalDAV and SOGo will report that the calendar collection only supports events.
Once done, simple refresh calendars (⌘R) or restart the Calendar application to be able to delete your calendar.