How to lock a textfield after it receives an input? (ControlP5)

I want the textfield to block once it receives a value.

I've seen there is a Textfield lock() method but I don't know how to use it.

import controlP5.*;

ControlP5 cp5;



color ccolor=color(250, random(100), random(100));

void setup() {
  size(700,400);

  PFont font = createFont("arial",20);

  cp5 = new ControlP5(this);

  cp5.addTextfield("Subject number")
     .setPosition(20,170)
     .setSize(200,40)
     .setFont(createFont("arial",20))
     .setAutoClear(true)
     .setColorLabel(ccolor);
     ;

  cp5.addBang("OK")
     .setPosition(240,170)
     .setSize(40,40)
     .getCaptionLabel().align(ControlP5.CENTER, ControlP5.CENTER)
     ;

  textFont(font);
}

void draw() {
  background(0);
  fill(255);

}



void controlEvent(ControlEvent theEvent) {
  if(theEvent.isAssignableFrom(Textfield.class)) {
    println("controlEvent: accessing a string from controller '"
            +theEvent.getName()+"': "
            +theEvent.getStringValue()
            );
  }
}