Stop rigidbody from sliding down "stairs".

EDIT I added your suggestion and it stops me, but like you said, I get frozen. Now this next part of script allows me to see when my WASD buttons are being pressed, and it worked without any issues.

 if(!Input.GetButton("Horizontal"))
     {    
         //Debug.Log("There is no horizontal input.");
         if(!Input.GetButton("Vertical"))
         {    
             //Debug.Log("There is no Vertical input.");
             moveInput = false;
         }
     }    
     //If there IS WASD input, moveInput = true.
     if(Input.GetButton("Horizontal") == true)
     {    
         //Debug.Log("There IS horizontal input.");
         moveInput = true;
     }
     if(Input.GetButton("Vertical") == true)
     {    
         //Debug.Log("There IS vertical input.");
         moveInput = true;
     }    

This next part stops me just fine, but it won't let me restart.

 if(collision.gameObject.tag =="StairsPlane")
     {
         //Debug.Log("In contact w/stairs plane.");
         if(moveInput == false)
             rigidbody.constraints = RigidbodyConstraints.FreezePosition;
         else
             rigidbody.constraints = RigidbodyConstraints.FreezeNone;
     }

I'm using a rigidbody for my character. My stairs consist of a ramp that the character "walks up". My issue is that he slides down the ramp when I stop moving. Can anyone offer me some assistance on how to stop this?

PlayerMovementScript.js
var walkAcceleration : float = 5; var walkAccelAirRacio : float = 0.1; var walkDeacceleration : float = 5; @HideInInspector var walkDeaccelerationVolx : float; @HideInInspector var walkDeaccelerationVolz : float;

 var cameraObject : GameObject;
 var maxWalkSpeed : float = 20;
 @HideInInspector
 var horizontalMovement : Vector2;
 var jumpVelocity : float = 20;
 @HideInInspector
 var grounded : boolean = false;
 var maxSlope : float = 60;
 
 //crouching
 var crouchRacio : float = 0.3;
 var transitionToCrouchSec : float = 0.2;
 @HideInInspector
 var crouchVelocity : float;
 var currentCrouchRacio : float = 1;
 @HideInInspector
 var originalLocalScaleY : float;
 @HideInInspector
 var crouchLocalScaleY : float;
 @HideInInspector
 var crouchingSpeed : float;
 @HideInInspector
 var maxWalkSpeedOrig : float;
 
 //end crouching and collision detection
 var collisionDetectionSphere : GameObject;
 
 //switch guns
 @HideInInspector
 var waitFrameForSwitchGuns : int = -1;
 var currentGun : GameObject;
 var distToPickUpGun : float =6;
 var throwGunUpForce : float = 100;
 var throwGunForwardForce : float = 300;
 
 //Pickup Ammo
 @HideInInspector
 var waitToPickupAmmo : float = 0;
 
 //Position for AIPathCell
 var currentCell : GameObject;
 
 
 function Awake ()
 {
     maxWalkSpeedOrig = maxWalkSpeed;
     currentCrouchRacio = 1;
     originalLocalScaleY = transform.localScale.y;
     crouchLocalScaleY = transform.localScale.y * crouchRacio;
     crouchingSpeed = maxWalkSpeed / 10;
 }
 
 function Update () 
 {
     waitFrameForSwitchGuns -= 1;
     waitToPickupAmmo -= Time.deltaTime;
     
     transform.localScale.y = Mathf.Lerp(crouchLocalScaleY, originalLocalScaleY, currentCrouchRacio);
     if(Input.GetButton("Crouch"))
         currentCrouchRacio = Mathf.SmoothDamp(currentCrouchRacio, 0, crouchVelocity, transitionToCrouchSec);
         maxWalkSpeed = crouchingSpeed;
         
     if(Input.GetButton("Crouch") == false && collisionDetectionSphere.GetComponent(CollisionDetectionSphereScript).collisionDetected == false)
         currentCrouchRacio = Mathf.SmoothDamp(currentCrouchRacio, 1, crouchVelocity, transitionToCrouchSec);
         maxWalkSpeed = crouchingSpeed;
     if(!Input.GetButton("Crouch"))
         maxWalkSpeed = maxWalkSpeedOrig;
 
     horizontalMovement = Vector2(rigidbody.velocity.x, rigidbody.velocity.z);
     if (horizontalMovement.magnitude > maxWalkSpeed)
     {
         horizontalMovement = horizontalMovement.normalized;
         horizontalMovement *= maxWalkSpeed;
     }
     rigidbody.velocity.x = horizontalMovement.x;
     rigidbody.velocity.z = horizontalMovement.y;
     
     if (grounded){
         rigidbody.velocity.x = Mathf.SmoothDamp(rigidbody.velocity.x, 0, walkDeaccelerationVolx, walkDeacceleration);
         rigidbody.velocity.z = Mathf.SmoothDamp(rigidbody.velocity.z, 0, walkDeaccelerationVolz, walkDeacceleration);
     }
     
     
     transform.rotation = Quaternion.Euler(0, cameraObject.GetComponent(MouseLookScript).currentYRotation, 0);
 
 //Is jumping, less movement control
     /*
     if(grounded)    
         rigidbody.AddRelativeForce(Input.GetAxis("Horizontal") * walkAcceleration * Time.deltaTime, 0, Input.GetAxis("Vertical") * walkAcceleration * Time.deltaTime);
     else
         rigidbody.AddRelativeForce(Input.GetAxis("Horizontal") * walkAcceleration * walkAccelAirRacio * Time.deltaTime, 0, Input.GetAxis("Vertical") * walkAcceleration * walkAccelAirRacio * Time.deltaTime);
     */
     
     if(Input.GetButtonDown("Jump") && grounded)
         rigidbody.AddForce(0, jumpVelocity,0);
 }
 
 function FixedUpdate ()
 {
     if(grounded)    
         rigidbody.AddRelativeForce(Input.GetAxis("Horizontal") * walkAcceleration, 0, Input.GetAxis("Vertical") * walkAcceleration);
     else
         rigidbody.AddRelativeForce(Input.GetAxis("Horizontal") * walkAcceleration * walkAccelAirRacio, 0, Input.GetAxis("Vertical") * walkAcceleration * walkAccelAirRacio);
     
 }
 
 
 function OnCollisionStay (collision: Collision)
 {
     for (var contact : ContactPoint in collision.contacts)
     {
         if (Vector3.Angle(contact.normal, Vector3.up) < maxSlope)
             grounded = true;
     }
 }
 
 function OnCollisionExit ()
 {
  grounded = false;
 }
 
 
 function OnTriggerStay (hitTrigger : Collider)
 {
 //Set current cell for Ai pathfinding
     if(hitTrigger.tag == "AIPathCell")
         currentCell = hitTrigger.gameObject;
 
 //Handles stairs up & down
     if (hitTrigger.transform.tag == "StairGoingUp")
 
         if (!Input.GetButton("Jump") && Vector3.Angle(rigidbody.velocity, hitTrigger.transform.forward) < 90)
 
             if (rigidbody.velocity.y > 0)
 
                 rigidbody.velocity.y = 0;
 
     if (hitTrigger.transform.tag == "StairGoingDown")
 
         if (!Input.GetButton("Jump") && Vector3.Angle(rigidbody.velocity, hitTrigger.transform.forward) < 90)
 
             rigidbody.AddForce(0,-100,0);
 
 

}