Remove "tags" from WP's admin UI.

gistfile1.php

<?php

add_action( 'admin_menu', function() {
	remove_submenu_page( 'edit.php', 'edit-tags.php?taxonomy=post_tag' ); // Remove menu link under "Posts"
});
add_action( 'admin_menu', function() {
	remove_meta_box( 'tagsdiv-post_tag', 'post', 'side'); // Remove metabox on add/edit post screen
});
add_filter( 'manage_posts_columns', function($columns) {
	unset( $columns['tags'] ); // Remove "Tags" column in post listing
	return $columns;
});