diff -u invite.orig/invite.module invite/invite.module
--- invite.orig/invite.module	2007-02-01 00:55:36.000000000 +0100
+++ invite/invite.module	2007-02-14 00:12:03.462143700 +0100
@@ -503,8 +503,9 @@
 function invite_delete($email) {
   global $user;
   $email = urldecode($email); // $email = rawurldecode(urlencode($email));
-  if ($email){
-    db_query("DELETE from {invite} WHERE email = '%s'",$email);
+  $account = user_load(array('mail' => $email));
+  if ($email && (!isset($account->uid) || variable_get('invite_allow_join_delete', 0))) {
+    db_query("DELETE from {invite} WHERE email = '%s'", $email);
     drupal_set_message(t('Invitation to @email has been deleted.', array('@email' => $email)));
     $args = array('email' => $email);
     module_invoke_all('invite', 'cancel', $args);