WordPress code to reserve some top level Permalink slugs.

fe-prevent-slug-conflict.php

<?php
/**
 * Prevent top level permalink slugs that will cause conflicts.
 * 
 * New rewrite slugs are introduced when CPTs or Custom Taxonomies are created.
 * This code adds a check when Pages or Posts are created, that their Permalink
 * does not conflict with one of the reserved top level slugs you define.
 *
 * In the case of a bad (i.e conflicting slug), WordPress appends a "-2" to
 * the permalink.
 */

add_filter( 'wp_unique_post_slug_is_bad_hierarchical_slug', 'fe_prevent_slug_conflict', 10, 4 );
add_filter( 'wp_unique_post_slug_is_bad_flat_slug', 'fe_prevent_slug_conflict', 10, 3 );

/**
 * Is this a top level permalink slug that conflicts with a reserved slug.
 *
 * @param bool $is_bad_slug Is the slug being passed in already marked as a bad slug.
 * @param string $slug The slug to be tested for uniqueness.
 * @param string $post_type The post type the slug is going to be assigned to.
 * @param int $post_parent_id (optional) When the slug is for a heirarchical post type,
 *                                       the post ID of the parent post.
 * @return bool Does this slug conflict with an existing slug or reserved slug.
 */
function fe_prevent_slug_conflict( $is_bad_slug, $slug, $post_type, $post_parent_id = 0 ) {

	$reserved_top_level_slugs = apply_filters(
		'fe_reserved_top_level_slugs',
		array('recipes',)
	);

	if (
		// Only check top level post slugs (i.e. not child posts).
		0 === $post_parent_id
		&& in_array( $slug, $reserved_top_level_slugs )
	) {
		$is_bad_slug = true;
	}
	return $is_bad_slug;
}