Customize Readonly WordPress Plugin: Only let superadmins make changes in the Customizer. All other admins can just preview changes. For them, the save UI is removed/disabled, and the Ajax request to save settings is blocked. The customize_readonly filter can be used to fine-tune which users are read-only.

customize-readonly.js

/*global wp, jQuery */
wp.customize.ReadOnly = ( function ( $, api ) {
	var self;

	self = {};

	self.init = function () {
		// Hide the save button
		$( '#save' ).hide();

		// Clean the saved state whenever it is dirty
		api.state( 'saved' ).bind( function( saved ) {
			if ( ! saved ) {
				this( true );
			}
		} );

	};

	api.bind( 'ready', function () {
		self.init();
	} );

	return self;
}( jQuery, wp.customize ) );

customize-readonly.php

<?php
/**
 * Plugin Name: Customize Readonly
 * Description: Only let superadmins make changes in the Customizer. All other admins can just preview changes. For them, the save UI is removed/disabled, and the Ajax request to save settings is blocked. The <code>customize_readonly</code> filter can be used to fine-tune which users are read-only.
 * Author: Weston Ruter, X-Team WP
 * Version: 0.1
 * Author URI: http://x-team.com/wordpress/
 * License: GPLv2+
 */

/**
 * Copyright (c) 2014 X-Team (http://x-team.com/)
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License, version 2 or, at
 * your discretion, any later version, as published by the Free
 * Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 */

class Customize_Readonly {

	function __construct() {
		add_action( 'customize_register', array( $this, 'init' ) );
	}

	/**
	 * Setup readonly mode on customize_register.
	 */
	function init() {
		if ( ! $this->is_user_readonly() ) {
			return;
		}

		add_action( 'customize_save', array( $this, 'block_customize_save' ), 1 );
		add_action( 'customize_controls_print_scripts', array( $this, 'enqueue_scripts' ) );
	}

	/**
	 * Register/enqueue customizer scripts.
	 */
	function enqueue_scripts() {
		wp_enqueue_script( 'customize-readonly', plugin_dir_url( __FILE__ ) . '/customize-readonly.js', array( 'jquery', 'customize-controls' ) );
	}

	/**
	 * Prevent customizer settings from being saved.
	 *
	 * customize_save action happens before any settings are saved. This is
	 * invoked during wp_ajax_customize_save
	 */
	function block_customize_save() {
		wp_send_json_error( 'readonly' );
	}

	/**
	 * Users can be made readonly to customize via the customize_readonly filter.
	 *
	 * @return bool
	 */
	function is_user_readonly() {
		return apply_filters( 'customize_readonly', ! is_super_admin(), wp_get_current_user() );
	}
}

$customize_readonly = new Customize_Readonly();