Disable a script from another script

Hello,

I have a problem: i can't disable a script from an other script. why? I am making a menu, and i want to stop my player moving, when it is on pause. Or is there an other way to do it?

 // the var 'P' = pause
 static var P=0;
 function Update () {
 // i did define the escape button! 
  if(Input.GetButtonDown("esc")){
     P=1;
  }
 }
 function OnGUI()
 {
  if (P==1){
            Screen.showCursor = true;
 
            GUI.Box(Rect(-45,0,2000,1000), "Pause");
            if(GUI.Button(Rect(910,300,100,30), "Resume")){P=0; Screen.showCursor = false;}
            if(GUI.Button(Rect(910,500,100,30), "Exit")){Application.Quit();}
 
   /*Error-->*/ gameObject.Find("FirstPersonControllerPrefab").FPSWalker = false;
   /*Error-->*/ gameObject.Find("FirstPersonControllerPrefab").FPSWalker.enable = false;
   /*Error-->*/ gameObject.Find("FirstPersonControllerPrefab").FPSWalker.false;
   /*Error-->*/ gameObject.Find("FirstPersonControllerPrefab").FPSWalker.active = false;
         
  }
 }

Please help! ASAP!!! :-/