How to pass on the user id(block user) after registration
The user registration form is generated by user_register_form(), and like normal Drupal forms, it can be altered via hook_form_alter or hook_form_FORM_ID_alter. When not explicitely set, the submit handler of a form is named form_id_submit(), however, in user_register_form, a submit handler is added to the #submit property of the form - user_register_submit().

In the function user_register_submit, you'll notice the following codes:

...
$form_state['user'] = $account;
$form_state['values']['uid'] = $account-&gt;uid;
...


which saves the new user object and ID in $form_state.

To access this after the registration form is submitted, simply add a custom submit handler via hook_form_alter() or hook_form_FORM_ID_alter(). Example:

/**
 * Implements hook_form_FORM_ID_alter().
 *
 * Alters the user registration form by adding a custom submit handler.
 */
function foo_form_user_register_form_alter(&amp;$form, &amp;$form_state) {
  $form['actions']['submit']['#submit'][] 'foo_user_register_form_submit';
}

/**
 * Custom submit handler for the user registration form.
 */
function foo_user_register_form_submit($form, &amp;$form_state) {
  $account = $form_state['user'];

  $uid = $form_state['values']['uid'];

  // Do something meaningful
}