How to hide posts completely

I thought about a couple of different approaches to this, most of which did not really work all together smoothly.

I think the best solution is to redirect the user to a login page if the category being requested is a blocked category. Here’s a code snippet that does that:

add_action( 'template_redirect', 'my_blocked_cat_redirect' );
function my_blocked_cat_redirect() {
	// get the category ID
	$cat_id = get_query_var( 'cat' );
	
	// if the category ID is the blocked category, redirect to login page
	if( $cat_id == 4 ) {
		wp_redirect( '/login/?redirect_to=' . get_category_link( $cat_id ) );
		exit();
	}
}

The first thing to note is that in order to redirect, we need to do that before headers are sent. template_redirect is one of the later actions where we have the post info or category ID and can still do this.

You will need to change the category ID that the if logic is testing for to match the ID of your blocked category.

Next, you will need to create your own correct redirect URL. The example uses the slug /login/ so if your login page is different, that will need to change.

The redirect link also sets up a ?redirect_to= parameter for the link so that the login page will redirect back to the originally requested category page. I used get_category_link() to build that value. This uses the category ID for the requested category.