diff --git a/pm_block_user/pm_block_user.module b/pm_block_user/pm_block_user.module
index b8c1e67206ec7f01b85839e5a94226820c5d2092..16a42d0b90bb0cbf407307c1b4d52217a72b72b5 100755
--- a/pm_block_user/pm_block_user.module
+++ b/pm_block_user/pm_block_user.module
@@ -161,12 +161,28 @@ function _pm_block_user_access($account) {
   if (!privatemsg_user_access('read privatemsg', $user)) {
     return FALSE;
   }
-  if (_pm_block_user_rule_exists($account, $user, PM_BLOCK_USER_DISALLOW_BLOCKING)) {
+  // Allow to unblock users that are already blocked but the user is now not
+  // allowed to block anymore.
+  if (_pm_block_user_rule_exists($account, $user, PM_BLOCK_USER_DISALLOW_BLOCKING) && !pm_block_user_has_blocked($account, $user)) {
     return FALSE;
   }
   return TRUE;
 }
 
+/**
+ * Checks if author is blocked by the recipient.
+ *
+ * @param $author
+ *   The user that would send a message.
+ * @param $recipient
+ *   The user that would receive the message.
+ * @return
+ *   TRUE if the recipient has blocked the author.
+ */
+function pm_block_user_has_blocked($author, $recipient) {
+  return db_query('SELECT 1 FROM {pm_block_user} WHERE author = :author AND recipient = :recipient', array(':author' => $author->uid, ':recipient' => $recipient->uid))->fetchField();
+}
+
 /**
  * Checks whether a rule exists for a given author, recipient and action.
  *
diff --git a/pm_block_user/pm_block_user.pages.inc b/pm_block_user/pm_block_user.pages.inc
index 692ad04336be57e315414459579f87a5e59f87b2..fc49edb2ce7656be7ba3e67c607eab13f591d615 100644
--- a/pm_block_user/pm_block_user.pages.inc
+++ b/pm_block_user/pm_block_user.pages.inc
@@ -25,11 +25,7 @@ function pm_block_user_form($form, $form_state, $author) {
     '#type' => 'value',
     '#value' => isset($_GET['destination']) ? $_GET['destination'] : 'messages/',
   );
-  $blocked = db_query('SELECT COUNT(recipient) FROM {pm_block_user} WHERE author = :author AND recipient = :recipient', array(
-    ':author' => $author->uid,
-    ':recipient' => $user->uid,
-  ))->fetchField();
-  if ($blocked) {
+  if (pm_block_user_has_blocked($author, $user)) {
     $form['block_action'] = array(
       '#type' => 'value',
       '#value' => 'unblock_user',
@@ -37,7 +33,7 @@ function pm_block_user_form($form, $form_state, $author) {
     return confirm_form($form,
       t('You have previously blocked "@author" from sending you any more messages. Are you sure you want to unblock this user?', array('@author' => $author->name)),
       isset($_GET['destination']) ? $_GET['destination'] : 'messages/',
-      t('This action cannot be undone.'),
+      '',
       t('Unblock @author', array('@author' => $author->name)),
       t('Cancel')
     );
@@ -99,10 +95,12 @@ function pm_block_user_list() {
     '#title' => t('Username'),
     '#autocomplete_path' => 'messages/user-name-autocomplete',
     '#description'        => t('Separate multiple names with commas.'),
+    '#required'          => TRUE,
   );
   $form['new']['submit'] = array(
     '#type' => 'submit',
     '#value' => t('Block user'),
+    '#validate' => array('pm_block_user_block_validate'),
     '#submit' => array('pm_block_user_block_submit'),
   );
 
@@ -139,22 +137,58 @@ function pm_block_user_list() {
 }
 
 /**
- * Submit callback for block user form.
+ * Validate user names.
  */
-function pm_block_user_block_submit($form, &$form_state) {
+function pm_block_user_block_validate($form, &$form_state) {
   global $user;
   list($accounts, $invalid) = _privatemsg_parse_userstring($form_state['values']['name']);
+  // Remove acocunts that can not be blocked.
   if (!empty($accounts)) {
-    $insert = db_insert('pm_block_user')->fields(array('author', 'recipient'));
-    foreach ($accounts as $account) {
-      $insert->values(array(
-        'author' => $account->uid,
-        'recipient' => $user->uid,
-      ));
-      drupal_set_message(t('!author has been blocked from sending you any further messages.', array('!author' => theme('username', array('account' => $account)))));
+    foreach ($accounts as $id => $account) {
+      // Check if the user can not be blocked because of a rule.
+      if (_pm_block_user_rule_exists($account, $user, PM_BLOCK_USER_DISALLOW_BLOCKING)) {
+        drupal_set_message(t('You are not allowed to block !account.', array('!account' => theme('username', array('account' => $account)))), 'warning');
+        unset($accounts[$id]);
+      }
+      // Check if the user is already blocked.
+      if (pm_block_user_has_blocked($account, $user)) {
+        drupal_set_message(t('You have already blocked !account.', array('!account' => theme('username', array('account' => $account)))), 'warning');
+        unset($accounts[$id]);
+      }
+      // Do not allow users to block themself.
+      if ($user->uid == $account->uid) {
+        drupal_set_message(t('You can not block yourself.'), 'warning');
+        unset($accounts[$id]);
+      }
     }
-    $insert->execute();
   }
+  // Display warning about invalid user names.
+  if (!empty($invalid)) {
+    drupal_set_message(t('The following users do not exist: @invalid.', array('@invalid' => implode(", ", $invalid))), 'warning');
+  }
+  // If there are no accounts left, display error.
+  if (empty($accounts)) {
+    form_set_error('name', t('You are either not allowed to block these users or the users do not exist.'));
+  }
+  else {
+    $form_state['valid_accounts'] = $accounts;
+  }
+}
+
+/**
+ * Submit callback for block user form.
+ */
+function pm_block_user_block_submit($form, &$form_state) {
+  global $user;
+  $insert = db_insert('pm_block_user')->fields(array('author', 'recipient'));
+  foreach ($form_state['valid_accounts'] as $account) {
+    $insert->values(array(
+      'author' => $account->uid,
+      'recipient' => $user->uid,
+    ));
+    drupal_set_message(t('!author has been blocked from sending you any further messages.', array('!author' => theme('username', array('account' => $account)))));
+  }
+  $insert->execute();
 }
 
 /**