user_readonly.module

<?php 
/**
 * Implementation of hook_form_alter().
 */
function user_readonly_form_alter(&$form, $form_state, $form_id) {
  global $user;
  $readonly_list = variable_get('user_readonly_list', array());
  if (array_key_exists($user->uid, $readonly_list)) {
    $form['#validate'][] = 'user_readonly_form_validate';
  }
  return $form;
}

/**
 * Implementation of hook_form_FORM_ID_validate().
 */
function user_readonly_form_validate(&$form, $form_state) {
  form_set_error('readonlymode', t("You are not allowed to change any data."));
}

function user_readonly_settings() {
  if (function_exists("jquery_ui_get_version")) {
    $ver = jquery_ui_get_version();
    if (strstr($ver, '1.7.')) {
      drupal_add_css(drupal_get_path('module', 'jquery_ui') . '/jquery.ui/themes/base/ui.all.css');
      drupal_add_css(drupal_get_path('module', 'user_readonly') . '/css/ui.multiselect.css');
      jquery_ui_add('ui.sortable');
      drupal_add_js(drupal_get_path('module', 'user_readonly') . '/js/ui.multiselect.js');
      drupal_add_js('$(document).ready(function (context) {
          $(".multiselect").multiselect();
      });', 'inline');
    }
  }
  
  $choice = new stdClass();
  $choice->option = array(0 => t('Anonymous'));
  $options[] = $choice;
    
  $result = db_query('SELECT uid, name FROM {users} WHERE uid > 1');
  while ($user = db_fetch_array($result)) {
    $choice = new stdClass();
    $choice->option = array($user['uid'] => $user['name']);
    $options[] = $choice;
  }

  $form['user_readonly_list'] = array(
    '#attributes' => array('class' => 'multiselect'),
    '#type' => 'select',
    '#title' => t('Read-only users'),
    '#default_value' => variable_get('user_readonly_list', array()),
    '#options' => $options,
    '#size' => 20,
    '#multiple' => TRUE,
    '#description' => t('Select which users have read-only access.'),
  );
  return system_settings_form($form);
}

function user_readonly_menu() {
  $items['admin/user/user_readonly'] = array(
    'title' => 'ReadOnly user',
    'description' => t('Select which users have read-only access.'),
    'page callback' => 'drupal_get_form',
    'page arguments' => array('user_readonly_settings'),
    'access arguments' => array('access administration pages'),
    'type' => MENU_NORMAL_ITEM,
   );
   return $items;
}