By default, this mod takes the first paragraph from the first topic to create the description. But you can use the Topic Descriptions mod with Optimus. It will help to fill more concrete descriptions. Or, if you are using the SMF 2.1.x, just enable "Allow a separate field for the topic description" option for that.
A different variant, open Class-Optimus.php, find
$row['body'] = parse_bbc($row['body'], false);
and replace with
$row['body'] = parse_bbc($row['body'], false);
$row['body'] = str_replace('Good afternon,<br />', '', $row['body']);
or with
$row['body'] = parse_bbc($row['body'], false);
$row['body'] = str_replace('Good afternon,<br /><br />', '', $row['body']);