To do this without coding will be difficult. Contact Form 7 is meant to send out emails after form submission, not much more. You need a callback function that you can use to implement your coupon logic.
One possible solution:
Install the Redirect Addon for Contact Form 7 and configure it to redirect to a local page, ie. “thankyou”. On this page, you could place a shortcode, which will provide you with the necessary callback function to retrieve a coupon code from the list and return it to the user.
Other than that, you could hook into wpcf7_before_send_mail, which has the benefit of being able to access the POST data (which will be gone after a redirect).
Yet another way would be to have your own shortcode within the mail body, ie. [coupon_code]. Here’s how you could use this to display the coupon code in the outgoing mail: https://wordpress.org/support/topic/custom-shortcode-in-emails/