class MyModelAdmin extends ModelAdmin {
...
   public static $record_controller_class = "MyModelAdmins_RecordController";
...
}

class MyModelAdmin_RecordController extends ModelAdmin_RecordController {
...
   public function EditForm() {
      $form = parent::EditForm();
      $fields = $form->Actions();
      $fields->removeByName('action_goBack');
      $form->setActions ($fields);
      return $form;
   }
...
}