How can I exclude posts with a specific custom field and value with this query?

$my_query = new WP_Query(  $args = array(
        // It should be in the first category of our post:
        'category__in' => array( $st_cat ),
        // Our post should NOT be in the list:
        'post__not_in' => array( $post->ID ),
        'posts_per_page' => 8,
        'orderby' => 'desc',
    ));

I already tried this withou sucess:

$my_query = new WP_Query(  $args = array(
        // It should be in the first category of our post:
        'category__in' => array( $st_cat ),
        // Our post should NOT be in the list:
        'post__not_in' => array( $post->ID ),
        'posts_per_page' => 8,
        'orderby' => 'desc',
         //extra code added without sucess
        'meta_query' => array(
        'key' => 'city',
        'value' => 'true',
        'compare' => 'NOT LIKE',
    )
));