Solution will:

BxDolClassifieds.php
function GenAnyBlockContent

if( !mysql_num_rows( $rBlogs ) )
return MsgBox(_t( '_Sorry, nothing found' ));

and at bottom

if ($sBlocks == '') $sBlocks = MsgBox(_t('_Sorry, nothing found'));
return $sBlocks;
}


so just provide 2 places here with
MsgBox(_t('_Sorry, nothing found'))