Modify the user role that is given to users that register during checkout
<?php
/*
 * Plugin Name: Easy Digital Downloads - Modify User Role on Checkout
 * Description: Modify the user role that is given to users that register during checkout
 * Author: Easy Digital Downloads
 * Author URI: https://easydigitaldownloads.com/
 * Version: 1.0
 */

function pw_edd_customer_user_role( $user_args, $user_data ) {
	
	// Set the role to the role you wish customers to have
	$user_args['role'] = 'customer';

	return $user_args;
}
add_filter( 'edd_insert_user_args', 'pw_edd_customer_user_role', 10, 2 );
To implement this snippet on your site, create a brand new, empty WordPress plugin and paste the above code inside.