Override a function

I need to override this function

Code:

abstract class Am_Helpdesk_Grid extends Am_Grid_Editable_Helpdesk
{
    public function renderSubject($record)
    {
        $url = Am_Di::getInstance()->helpdeskStrategy->assembleUrl(array(
            'page_id' => 'view',
            'action' => 'view',
            'ticket' => $record->ticket_mask
        ), 'inside-pages');

        return sprintf('<td><a href="%s" target="_top">%s</a></td>',
            $url,
            Am_Controller::escape($record->subject)
        );
    }
}

to remove the target="_top"

How do I go about doing this? Right now I have hacked the code base but I really dislike that solution.

Thanks