Use Screen.showCursor:

Javascript:

function Update() {
Screen.showCursor = false;
}

C# (you'll need to make sure your script is named hideCursor.cs):

using UnityEngine;
using System.Collections;

public class hideCursor : MonoBehaviour {
 public void Awake() {
  Screen.showCursor = false;
 }
}

Boo (like C#, make sure your script is named hideCursor.boo):

import UnityEngine
import System.Collections

class hideCursor(MonoBehaviour):

public def Awake():
Screen.showCursor = false