This is from an Elegant Themes blog post, where they also give some other method options: https://www.elegantthemes.com/blog/divi-resources/how-to-remove-sidebars-from-divi).

/*** Take out the divider line between content and sidebar ***/
  #main-content .container:before {background: none;}

/*** Hide Sidebar ***/
  #sidebar {display:none;}

/*** Expand the content area to fullwidth ***/
  @media (min-width: 981px){
  #left-area {
  width: 100%;
  padding: 23px 0px 0px !important;
  float: none !important;
  }
  }

Where to paste the CSS snippet

Navigate to Divi>Theme Options and at the bottom of the default “General” tab, paste the CSS snippet into the “Custom CSS” box and save changes.

Child Themes

We recommend using a child theme. It is your insurance against losing your custom work in updates.

Click here for instructions on adding a basic Divi child theme with a stylesheet (for custom CSS).

Click here to learn more about child themes and getting the most out of the Divi theme.