Hi,
as I know there is no such option built in. If you just want to do this once and you can write simple code in PHP using custom crits.
To get all companies where city value isn't "bangkok" (NOT "bangkok", "Bangkok", " Bangkok XYZ") - use below crits.
$crits = array('!~city' => '%bangkok%');
To get all companies without city set use below crits.
$crits = array('city' => '');
And here how to get companies records.
Utils_RecordBrowserCommon::get_records('company', $crits);
Regards,
Adam