Hiding WordPress Categories: In the following simple solution, I am going to show you How to keep a selection of WordPress Categories from displaying in your sidebar on your WordPress pages and posts. This is very useful if you have a lot of categories, or have changed the content of your blog, have been indexed for the content and no longer wish to use or display specific categories on your WordPress site. Upon completion of this tutorial, the permalink structure of the hidden categories are still made available to the search engines, however, the categories you have chosen to hide are no longer displayed on your blog to the visitor.
Locate which categories you would like to hide from displaying in the sidebar. Category ID's can be found from the WordPress Admin Panel under Manage-> Categories. In WordPress 2.5 and beyond, simply hover over a category and the cat_ID=# will be listed at the bottom of your browser window.
Using an FTP client, FTP to your WordPress installation
Navigate to wp-includes/widgets.php and copy widgets.php to your desktop
Open widgets.php with a text editor and find the following line: