Disabling the Z axis?

Hello,

i have Created a Sphere with a texture on it.

The texture shows multiple pictures and the user is able to rotate the sphere left and right, and up and down...is there a way i can lock the Z axis' rotation to zero, so my rotation doesn't become threedimensional?

here's my script

 var kugel : GameObject;
 
 
 
 function Start()
 {
     
     
 
 
 }    
     
 
 
 
 
 
 function Update () {
     
         if(Input.GetKey("a"))
     {
         kugel.transform.Rotate(0,35*Time.deltaTime,0);
     }
     
     if(Input.GetKey("d"))
     {
         kugel.transform.Rotate(0,-35*Time.deltaTime,0);
     }
     
     if(Input.GetKey("w"))
     {
         kugel.transform.Rotate(35*Time.deltaTime,0,0);
     }
     if(Input.GetKey("s"))
     {
         kugel.transform.Rotate(-35*Time.deltaTime,0,0);
     }
     
 
 }