The Divi Theme's contact form module displays a contact from consisting of input fields (such as the message textbox) followed by a submit button (and optional captcha). If you'd like to adjust the space between the input fields and submit button / captcha, you can do so with this CSS:
Related Post: Adding CSS to the Divi Theme
The default value used by Divi for the top-margin is -1.5%. Decreasing it will reduce the gap between the input fields and submit button (a top-margin of around -4% seems to get rid of the gap entirely on an uncustomized contact form). Increasing it, e.g. to 0% or higher, will move the button down and increase the gap between it and the input fields.