Exclude a ‘portfolio’ custom category?

Please take a look at this example;

$args = array(
    'post_type' => 'post',
    'tax_query' => array(
        'relation' => 'AND',
        array(
            'taxonomy' => 'movie_genre',
            'field' => 'slug',
            'terms' => array( 'action', 'comedy' )
        ),
        array(
            'taxonomy' => 'actor',
            'field' => 'id',
            'terms' => array( 103, 115, 206 ),
            'operator' => 'NOT IN' //you must set the operator to NOT IN
        )
    )
);
$query = new WP_Query( $args );

See WP_Query in the Codex for more details;

http://codex.wordpress.org/Class_Reference/WP_Query#Taxonomy_Parameters