zarathoustra

I just found a better solution, combining the two first solutions.
In a file named functions.php located in my template folder, I copied/pasted the functions get_the_category_list() and get_the_category(), I renamed these functions in get_the_category_list_excludeCat() and get_the_category_excludeCat() with two little changes :

• changed the function get_the_category into get_the_category_excludeCat() in the function get_the_category_list_excludeCat()
• excluded a category in the function get_the_category_excludeCat()

My english is sooo bad, so check the following code, I think it would be more clear.

function get_the_category_list_excludeCat($separator = '', $parents='') {
	global $wp_rewrite;
        // change the function get_the_category into get_the_category_excludeCat()
	$categories = get_the_category_excludeCat();
	if (empty($categories))
		return apply_filters('the_category', __('Uncategorized'), $separator, $parents);

	$rel = ( is_object($wp_rewrite) && $wp_rewrite->using_permalinks() ) ? 'rel="category tag"' : 'rel="category"';

	$thelist = '';
	if ( '' == $separator ) {
		$thelist .= '<ul class="post-categories">';
		foreach ( $categories as $category ) {
			$thelist .= "\n\t<li>";
			switch ( strtolower($parents) ) {
				case 'multiple':
					if ($category->parent)
						$thelist .= get_category_parents($category->parent, TRUE);
					$thelist .= '<a href="' . get_category_link($category->term_id) . '" title="' . sprintf(__("View all posts in %s"), $category->name) . '" ' . $rel . '>' . $category->name.'</a></li>';
					break;
				case 'single':
					$thelist .= '<a href="' . get_category_link($category->term_id) . '" title="' . sprintf(__("View all posts in %s"), $category->name) . '" ' . $rel . '>';
					if ($category->parent)
						$thelist .= get_category_parents($category->parent, FALSE);
					$thelist .= $category->name.'</a></li>';
					break;
				case '':
				default:
					$thelist .= '<a href="' . get_category_link($category->term_id) . '" title="' . sprintf(__("View all posts in %s"), $category->name) . '" ' . $rel . '>' . $category->cat_name.'</a></li>';
			}
		}
		$thelist .= '</ul>';
	} else {
		$i = 0;
		foreach ( $categories as $category ) {
			if ( 0 < $i )
				$thelist .= $separator . ' ';
			switch ( strtolower($parents) ) {
				case 'multiple':
					if ( $category->parent )
						$thelist .= get_category_parents($category->parent, TRUE);
					$thelist .= '<a href="' . get_category_link($category->term_id) . '" title="' . sprintf(__("View all posts in %s"), $category->name) . '" ' . $rel . '>' . $category->cat_name.'</a>';
					break;
				case 'single':
					$thelist .= '<a href="' . get_category_link($category->term_id) . '" title="' . sprintf(__("View all posts in %s"), $category->name) . '" ' . $rel . '>';
					if ( $category->parent )
						$thelist .= get_category_parents($category->parent, FALSE);
					$thelist .= "$category->cat_name</a>";
					break;
				case '':
				default:
					$thelist .= '<a href="' . get_category_link($category->term_id) . '" title="' . sprintf(__("View all posts in %s"), $category->name) . '" ' . $rel . '>' . $category->name.'</a>';
			}
			++$i;
		}
	}

	return apply_filters('the_category', $thelist, $separator, $parents);
}

function get_the_category_excludeCat($id = false) {
	global $post, $term_cache, $blog_id;

	$id = (int) $id;
	if ( !$id )
		$id = (int) $post->ID;

	$categories = get_object_term_cache($id, 'category');
	if ( false === $categories )
		$categories = wp_get_object_terms($id, 'category');

	//exclude cat 3 :
	unset($categories[3]);		

	if ( !empty($categories) )
		usort($categories, '_usort_terms_by_name');
	else
		$categories = array();

	foreach(array_keys($categories) as $key) {
		_make_cat_compat($categories[$key]);
	}

	return $categories;
}