It doesn’t appear possible to dequeue the related.css stylesheet.
// Dequeue YARP CSS Style Sheet
function crunchify_dequeue_header_styles() {
wp_dequeue_style( 'yarppRelatedCss' );
}
add_action( 'wp_enqueue_scripts', 'crunchify_dequeue_header_styles', 99 );
since the stylesheet is enqueued during the_content… see source: https://github.com/wp-plugins/yet-another-related-posts-plugin/blob/d5053e706c7aff9dc55c0ce5ea33a1e3899b1414/classes/YARPP_Core.php#L1044