delete a category from the sidebar

sidebar have this html:

<h2>Categories</h2>
<ul>
<li><a href="http://localhost/wordpress-it_IT-2.2.1/category/categ_new/"
>categ_new</a>
</li>
<li><a href="http://localhost/wordpress-it_IT-2.2.1/category/my_category/"
>my_category</a>
</li>
</ul>

I tried with this code:

function foo_bar($content, $category = null) {
  if($category->cat_name!='my_category') return $content;
}
add_filter('list_cats', foo_bar, 10, 2);

but I get:

<h2>Categories</h2>
<ul>
<li><a href="http://localhost/wordpress-it_IT-2.2.1/category/categ_new/"
>categ_new</a>
</li>
<li><a href="http://localhost/wordpress-it_IT-2.2.1/category//"
></a>
</li>
</ul>

I’v used this plugin too: http://djz.hu/blog/posts/372
but i still see the category in sidebar