Index: modules/forum/forum.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/forum/forum.module,v
retrieving revision 1.572
diff -u -p -r1.572 forum.module
--- modules/forum/forum.module	8 Aug 2010 12:47:00 -0000	1.572
+++ modules/forum/forum.module	8 Aug 2010 14:15:58 -0000
@@ -622,6 +622,20 @@ function forum_form_alter(&$form, $form_
       $requested_forum_id = arg(3);
       $form['taxonomy_forums'][$langcode]['#default_value'] = is_numeric($requested_forum_id) ? $requested_forum_id : NULL;
     }
+    // Do not allow posting into containers.
+    $containers = variable_get('forum_containers', array());
+    $options = $form['taxonomy_forums'][$langcode]['#options'];
+    $forums = array();
+    foreach ($options as $tid => $name) {
+      if (in_array($tid, $containers)) {
+        // Array options are displayed as OPTGROUP so user can't select them.
+        $forums[check_plain($name)] = array();
+      }
+      else {
+      	$forums[$tid] = $name;
+      }
+    }
+    $form['taxonomy_forums'][$langcode]['#options'] = $forums;
   }
 }