Sometimes, you may want to loop through all the WordPress categories, but excluding some of them.
This is useful if you are organizing your content in categories and keeping some of them as “private”, in order to have more flexibility when rendering them.

This little snippet of code must be copied&pasted in your functions.php file, and it will be called every time you are inside the WordPress loop, thus transparently altering its functionality. Obviously, you have to update the code of the category ID with the one you want: in this example is ID=40.

// No category “40” on the homepage
function preventHomepageTips($query) {
if($query->is_home() && $query->is_main_query()) {
$query->set('cat', '-40'); // 40 is the category ID we want to exclude
}
}
add_action('pre_get_posts', 'preventHomepageTips');


The original author of this snippet is David Walsh. He has also created a little plugin that does just this, be sure to check it out.