DMZ 1.7.1 (DataMapper OverZealous Edition)

[eluser]bEz[/eluser]
Why not run a routine that will simply assign "NULL" (on-demand) for a particular {USER} without using a text field.

Code:

$someObject = new aModel($this->input->post('user_id');
if ( $someObject->exists() ) {
  $someObject->last_name = NULL;
  $someObject->save();
  .
  .   // log/notify results of save routine
  .

} else {
. . .  // log/notify bad user id
}

You may also need to remove validation ("required") from the last name field, or turn-off validation for this particular routine assignment...