--- privatemsg.module	2008-03-20 13:43:52.000000000 +0800
+++ privatemsg.module	2008-03-20 13:52:43.000000000 +0800
@@ -318,7 +318,7 @@ function privatemsg_user($type, &$edit, 
              'title' => t('Send private message to @name', array('@name' => $account->name)))),
             'class' => 'send-message');
         }
-        if ($account->uid != $user->uid) {
+        if ($account->uid != $user->uid && $account->uid != 1) {
           if (!privatemsg_user_blocked($account->uid) ) {
             $return[t('Private messages')][] = array(
               'value' => l(t('Block messages'), 'privatemsg/block/'. $account->uid, array(
@@ -487,7 +487,10 @@ function privatemsg_user_blocked($author
 function privatemsg_block_user($author, $recipient = NULL) {
   global $user;
   $recipient = ($recipient) ? $recipient : $user->uid;
-  if (!privatemsg_user_blocked($author, $recipient)) {
+  if ($author == 1) {
+    drupal_set_message(t('You cannot block this user'));
+  }
+  else if (!privatemsg_user_blocked($author, $recipient)) {
     db_query("INSERT INTO {privatemsg_block_user} (author, recipient) VALUES (%d, %d)", $author, $recipient);
     drupal_set_message(t('User has been blocked'));
   }