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To delete them all at once, you'd have to disable Calendars in your email account, and not use it again, or, in the Calendar app, delete the Calendar/unmark it.
Supposedly that's not exactly what you want to do.
Maybe you could give more information so that we're able to help you - is it an iCloud calendar, or a different account? Do you intend to use that calendar again afterwards? Is your software up to date? The latest iOS is 7.1.1.