You might not be able to "disable paste" as such (without hosting the Flash control somehow yourself, say, in a Windows app, or in a browser extension of some sort), but you can certainly make a pretty good guess about the way someone's using the app with a little timer-based math. Here's a (super-)rough example of a Flex app illustrating what I mean:
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" xmlns:local="*" creationComplete="this_creationComplete(event)">
<mx:Script>
<![CDATA[
private var timer:Timer;
import flash.events.Event;
private function this_creationComplete(event:Event):void
{
timer = new Timer(1000);
timer.addEventListener(TimerEvent.TIMER, timer_tick);
timer.start();
}
private function timer_tick(event:TimerEvent):void
{
var elapsedTimeInMinutes:Number = timer.currentCount / 60;
var averageWordLength:Number = 4;
var humanlyPossible:Number = 200;
var thisPersonsSpeed:Number = (txtTest.text.length / averageWordLength) / elapsedTimeInMinutes;
if (thisPersonsSpeed > humanlyPossible)
{
txtSpeed.text = "Wait, " + Math.floor(thisPersonsSpeed).toString() + " words per minute? This clown is probably cheating.";
txtTest.enabled = false;
timer.stop();
}
else
{
txtSpeed.text = "Currently typing " + Math.floor(thisPersonsSpeed).toString() + " wpm. Hurry up! Faster!";
}
}
]]>
</mx:Script>
<mx:VBox>
<mx:TextArea id="txtTest" width="600" height="300" />
<mx:Text id="txtSpeed" />
</mx:VBox>
</mx:Application>
Essentially, it's just a timer that calculates words per minute; if that number exceeds a certain threshold, the timer stops, and the form disables.
Sure, it's not iron-clad, and if I were implementing it myself, I'd layer in some additional timing-oriented safeguards (e.g., stopping the timer after periods of inactivity, etc.), but it should illustrate the point. I'm sure there are other solutions, but something simple like this may work out well-enough for you.
Update: A couple of folks have mentioned Event.PASTE, which would work, but doesn't exist in ActionScript 2 / Flash Player 9. Provided you were able to ensure Flash Player 10 and could script in ActionScript 3, that'd be another option.