At first glance it seems as though there are no hooks or filters in WordPress for preventing a post/page deletion. But you can do it by filtering user_has_cap (short for user has capability). This is a very powerful filter, and you can use it to block almost anything in WordPress. It has three parameters:

• $allcaps (an array of all the capabilities, each one set to true or false)
• $caps (an array of the capabilities being requested by the current operation)
• $args (an array of arguments relevant to this operation).

When a post is being deleted, $args is set to array ('delete_post', $user_id, $post_id). The capabilities required to allow the deletion are stored in the array $caps, and will vary depending on what type of post is being deleted (e.g. ‘delete_published_posts’). Each capability in $caps corresponds to an item in $allcaps. To prevent the post being deleted, all we need to do is modify $allcaps by setting one of the values listed in $caps to false (e.g. $allcaps[$caps[0]] = false).

As an example, the following code prevents the last published page of a site being deleted.

add_filter ('user_has_cap', 'wpcs_prevent_last_page_deletion', 10, 3);

function wpcs_prevent_last_page_deletion ($allcaps, $caps, $args) {
	global $wpdb;
	if (isset($args[0]) && isset($args[2]) && $args[0] == 'delete_post') {
		$post = get_post ($args[2]);
		if ($post->post_status == 'publish' && $post->post_type == 'page') {
			$query = "SELECT COUNT(*) FROM {$wpdb->posts} WHERE post_status = 'publish' AND post_type = %s";
			$num_posts = $wpdb->get_var ($wpdb->prepare ($query, $post->post_type));
			if ($num_posts < 2)
				$allcaps[$caps[0]] = false;
		}
	}
	return $allcaps;
}