How do I remove the post format meta box?

That gets added because your theme supports post-formats.

You can either use a plugin (or a child theme’s functions.php) file to hook into after_setup_theme late and remove theme support:

<?php
add_action('after_setup_theme', 'wpse65653_remove_formats', 100);
function wpse65653_remove_formats()
{
   remove_theme_support('post-formats');
}

Or look for the the line in your theme’s functions.php file:

add_theme_support('post-formats');

and remove it.

The first option is a better bet. Stick it in a plugin and it will be there for the next theme you use as well.