Remove from Posts sub menu in left side WordPress Admin Menu:
function unbox_change_menus () {
remove_submenu_page( 'edit.php', 'edit-tags.php?taxonomy=post_tag' );
}
add_action('admin_menu', 'unbox_change_menus');
Remove Tags column from List of Posts in Admin:
add_filter('manage_posts_columns', 'unbox_custom_columns_posts');
function unbox_custom_columns_posts( $defaults ) {
unset($defaults['tags']); //remove tags
return $defaults;
}
Remove Tags Side Box when editing/creating post:
function unbox_remove_my_post_metaboxes() {
remove_meta_box( 'tagsdiv-post_tag', 'post', 'side');
}
add_action('admin_menu', 'unbox_remove_my_post_metaboxes');