If I'm not wrong, the string is:
$post['date'] = $txt['last_post'] . ' ' . $txt['by'] . ' <strong>' . $post['poster']['link'] . ' </strong> - ' . $post['html_time'];
I'm surprised there is still some use for the $txt['by'] particle, I thought I removed all of them...
That said, I feel it doesn't even make sense to have a "last post by" kind of string: the one showed can or cannot be the last message of the topic (the list is of recent messages) and can or cannot be the last message of the member, so I would change it altogether to... for example:
"Message by {author} at {time}"