How to disable “insert into post” only when selecting the Featured Image?

You can do this via,

add_filter('image_send_to_editor', 'restrict_images');

function restrict_images($html){

  return false;

}

…now consider the above function primitive, because it will restrict images for all post types. We can modify that on a post_type by post_type basis like so,

add_filter('image_send_to_editor', 'restrict_images');

function restrict_images($html){

    // check if its the admin trying to insert image, if so, quickly
    // return the image to the editor and do not run remainder of function
    if(current_user_can('activate_plugins'))
    return $html;

    // global $post and $wp_query not available so we use $_POST['key']
    $post_id    = $_POST['post_id'];

    // with post ID in hand we now get current post_type       
    $post_type  = get_post_type($post_id);

    // define list of our restricted post_types
    $restricted = array('post', 'page');

    // check current post_type against array of restricted post_types
    if (in_array($post_type, $restricted)) {

        // if our current post_type as returned from get_post_type is in array 
        // we return false to void inserting image into post editor
        return false;

    } else {

        // if our post_type does not exist in restricted array, carry on as normal
        return $html;
    }
}