With older versions of WPSEO (v1 ~ 2), you can remove the filter that it applies with the following code:
add_action( 'init', function() {
global $wpseo_front;
remove_filter( 'wp_title', array( $wpseo_front, 'title' ), 15 );
}, 20 );
WPSEO (~ 3 +) WordPress (4.4+) () :
add_action( 'init', function () {
$wpseo_front = WPSEO_Frontend::get_instance();
remove_filter( 'pre_get_document_title', array( $wpseo_front, 'title' ), 15 );
remove_filter( 'wp_title', array( $wpseo_front, 'title' ), 15 );
} );