Rigidbody movement walls climb disable

Hello i have a problem. I have maked own rigidbody movement script, and it have air acceleration. So the player can jump on the walls and it can stay on the walls. How i can stop it easily. I have tried physicmaterials what have 0 friction but it didn't work. here is the script :

 var speed = 10.0;
 var gravity = 10.0;
 var maxVelocityChange = 10.0;
 var canJump = true;
 var jumpHeight = 2.0;
 var airspeed = 8.0;
 private var truespeed = 0.0;
 private var grounded = false;
  
 @script RequireComponent(Rigidbody, CapsuleCollider)
  
 function Awake ()
 {
     rigidbody.freezeRotation = true;
     rigidbody.useGravity = false;
 }
  
 function FixedUpdate ()
 {
 
     // Calculate how fast we should be moving
         var targetVelocity = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
         targetVelocity = transform.TransformDirection(targetVelocity);
         targetVelocity *= truespeed;
  
         // Apply a force that attempts to reach our target velocity
         var velocity = rigidbody.velocity;
         var velocityChange = (targetVelocity - velocity);
         velocityChange.x = Mathf.Clamp(velocityChange.x, -maxVelocityChange, maxVelocityChange);
         velocityChange.z = Mathf.Clamp(velocityChange.z, -maxVelocityChange, maxVelocityChange);
         velocityChange.y = 0;
         rigidbody.AddForce(velocityChange, ForceMode.VelocityChange);
     
     if(grounded == false){
         truespeed = airspeed;
      
     }
     if (grounded == true)
     {
         truespeed = speed;
         
         // Jump
         if (canJump && Input.GetButton("Jump"))
         {
             rigidbody.velocity = Vector3(velocity.x, CalculateJumpVerticalSpeed(), velocity.z);
         }
     }
  
     // We apply gravity manually for more tuning control
     rigidbody.AddForce(Vector3 (0, -gravity * rigidbody.mass, 0));
  
     grounded = false;
 }
  
 function OnCollisionStay (hit : Collision)
 {
     grounded = true;    
     if(hit.gameObject.tag == "friction"){
          transform.parent = hit.transform ; 
      }else{
          transform.parent = null;
  }
 }
  
 function CalculateJumpVerticalSpeed ()
 {
     // From the jump height and gravity we deduce the upwards speed 
     // for the character to reach at the apex.
     return Mathf.Sqrt(2 * jumpHeight * gravity);
 }