I'm guessing it's because you used order_by when it should actually be orderby - that order_by wasn't just a typo in the question, right?
And also, if the meta value is a date, then you should also set the meta_type parameter to DATE so that the sorting works as expected.
So try with:
$the_query = new WP_Query( array(
'post_type' => 'event',
It didn't work because your tax query is missing the operator which should be set to NOT IN. So just add it like so:
'tax_query' => array(
'taxonomy' => 'post_format',
'field' => 'slug',
'terms' => array( 'post-format-' . $post_format ),
'operator' => 'NOT IN',
And please, avoid using ...