functions.php

<?php

/**
 * Only display Featured Image on archive
 * 
 * By default, Genesis will use the first attached image if no featured 
 * image is provided. This limits the archive to only show featured images.
 *
 * @param string $output
 * @param array $args
 * @param object $post
 * @return string $output
 */
function be_archive_featured_image_only( $output, $args, $post ) {
  if( 'archive' == $args['context'] && !has_post_thumbnail( $post->ID ) )
		$output = '';
	return $output;
}
add_filter( 'genesis_pre_get_image', 'be_archive_featured_image_only', 10, 3 );