How do I stop predictive text in username text input field?

You can use StageText instead of a regular TextField object. I just created a quick text of using StageText and tested it on my Moto X. One of the properties of StageText is "autoCorrect". I use the Google Keyboard and it still gave suggestions for autocorrect but it didnt automatically fix the "broken" words for me. Below is the code I used for a quick test I did on my phone.

import flash.text.StageText;
import flash.geom.Rectangle;

var textfield:StageText = new StageText();
textfield.autoCorrect = false;
textfield.fontSize = 20;
textfield.stage = this.stage;// must set to make visible. Set to "null" to hide
textfield.viewPort = new Rectangle(10, 10, this.stage.stageWidth - 20, this.stage.stageHeight - 20);

But if you absolutely had to use a TextField, I did think of a possibly ugly solution though. Create two textfields and have them be the same width/height and stacked on top of each other. Set the top one to be of type DYNAMIC and have its mouseEnabled set to false. The bottom one should be set to type INPUT, have its alpha set to 0, and add a listener for the Event.CHANGE event. When that event fires, copy the text from the INPUT textfield to the DYNAMIC textfield. The biggest downside to this is that the user can't tap to change the cursor location accurately or see where the cursor is. I tested this and it does work, but is not the ideal solve.