Exclude a category of posts in author template

Never use query_posts, use pre_get_posts to modify any main query.

function wpd_exclude_author_category( $query ) {
    if ( $query->is_author() && $query->is_main_query() ) {
        $query->set( 'category__not_in', array( 42 ) );
    }
}
add_action( 'pre_get_posts', 'wpd_exclude_author_category' );