how to hide a category

I wanna know how to hide a category from the a single post or in the index page
see when I wanna hide a category from the side bar I use the code
<?php wp_dropdown_categories(‘exclude=xx’); ?>
or what ever the categories shape but I mean the thing exclude=xx and xx is the ID of the category
so what it the code for the line after the post’s title ???
what are the words I need to edit in the code <?php the_category(‘, ‘) ?>