Limiting the rotation based on the angle of the player mesh

Hello:

I'm using the code below for rotation of the player mesh. I would like to know how to disable the rotation when you have in your inputs a new position that is an angle like walking backwards. I'm very new to Unity and the classes for position management.

 using UnityEngine;
 using System.Collections;
 
 public class PlayerMovement : MonoBehaviour {
 
     public float turnSmoothing = 15f;   // A smoothing value for turning the player.
     public float speedDampTime = 0.1f;  // The damping for the speed parameter
     public float translationSpeed = 2f;
     public float reverseAngle = 50f;
 
     void FixedUpdate()
     {
         // Cache the inputs.
         float h = Input.GetAxis("Horizontal");
         float v = Input.GetAxis("Vertical");
 
         MovementManagement(h, v);
     }
 
 
     void MovementManagement(float horizontal, float vertical)
     {
         if (horizontal != 0f || vertical != 0f)
         {
             // Create a new vector of the horizontal and vertical inputs.
             Vector3 targetDirection = new Vector3(horizontal, 0f, vertical);
 
             // Create a rotation based on this new vector assuming that up is the global y axis.
             Quaternion targetRotation = Quaternion.LookRotation(targetDirection, Vector3.up);
 
             // Create a rotation that is an increment closer to the target rotation from the player's rotation.
             Quaternion newRotation = Quaternion.Lerp(rigidbody.rotation, targetRotation, turnSmoothing * Time.deltaTime);
 
 
             //if (angle < 90) {
                 rigidbody.MoveRotation(newRotation);
                 //Debug.Log("rotating" + horizontal);
             //}   
 
             rigidbody.AddForce(new Vector3(horizontal*translationSpeed, 0f, vertical*translationSpeed));
         }
     }
 
 
 }

Thanks in advance,

Rayner