Specify meta boxes that should be hidden by default for new users.

functions.php

<?php

add_filter( 'default_hidden_meta_boxes', 'custom_default_hidden_screen_options', 10, 2 );

function custom_default_hidden_screen_options( $hidden, $screen ) {
	
	$hide_these = array( 
		'dashboard_primary',
		'postcustom',
		'trackbacksdiv',
	);
	
  return array_merge( $hidden, $hide_these );
}