Hide Subcategories in WordPress 3.0

Hi,
I would like to apply different style class to category navigation.So, I am using a custom function to display all categories.My script as follows

function your_list_categories($args){
	$defaults = array(
		'show_option_all' => '', 'show_option_none' => __('No categories'),
		'orderby' => 'name', 'order' => 'ASC',
		'show_last_update' => 0, 'style' => 'list',
		'show_count' => 0, 'hide_empty' => 1,
		'use_desc_for_title' => 1, 'child_of' => 0,
		'feed' => '', 'feed_type' => '',
		'feed_image' => '', 'exclude' => '',
		'exclude_tree' => '', 'current_category' => 0,
		'hierarchical' => true,
		'echo' => 1, 'depth' => 0,
		'taxonomy' => 'category'
	); 

   	$r = wp_parse_args( $args, $defaults );

	$categories = get_categories( $r );

    $output = '';

    foreach ($categories as $category) {
        if($category->name == "BEGINNER'S LESSONS"){
             $output .= '<li><a class="beginner" href="' . get_category_link( $category->term_id ) . '" title="' . sprintf( __( "View all posts in %s" ), $category->name ) . '" ' . '>' . $category->name.'</a></li>';
        } else {
          	$output .= '<li><a href="' . get_category_link( $category->term_id ) . '" title="' . sprintf( __( "View all posts in %s" ), $category->name ) . '" ' . '>' . $category->name.'</a></li>';
        }
    }
    echo $output;

}
add_filter('wp_list_categories', 'your_list_categories');

Invoking this function from my header.php
as follows

<?php
	$args=array(
	  'orderby' => 'name',
	  'depth'  => 1,
	  'hierarchical' => 1,
	  'order' => 'DESC'
	  );
	your_list_categories($args);
?>

Here how can I hide all subcategories.Please help me.