How to clamp rotation of an object?

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PLayermoves : MonoBehaviour
 {
     public Rigidbody rg;
     public float forwarforce = 0.000001f;
     public float sidewaysforce = 0.00001f;
     public float upwardsforce = 0.000005f;
     // Update is called once per frame
     void FixedUpdate()
     {
         rg.useGravity = true;
         if (Input.GetKey("w"))
         {
             rg.AddForce(0 , 0, forwarforce * Time.deltaTime);
         }
         if (Input.GetKey("d"))
         {
             rg.AddForce(sidewaysforce * Time.deltaTime, 0, 0);
         }
         if (Input.GetKey("a"))
         {
             rg.AddForce(-sidewaysforce * Time.deltaTime, 0, 0);
         }
         if (Input.GetKey("s"))
         {
             rg.AddForce(0, 0, -forwarforce * Time.deltaTime);
         }
         if (Input.GetKey("space"))
         {
             rg.AddForce(0, upwardsforce * Time.deltaTime, 0 );
         }
 
       
     }
 }