How do I stop a velocity only on the X axis?

I am making a character that moves in a 3D world with 2D mechanics.
They slip and slide about using the code below. I want it so that when the player lets go of the A/D keys it quickly brings them to a stop on the X-axis. I don’t want to use dampen on the Rigidbody because that will affect both Jumping and falling.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerMove : MonoBehaviour
    public float forceMult = 100;
    public float GravityMult = 0.5f; // Gravity
    private Rigidbody rb; //Rigidbody

    public bool JumpReady = true; //Jump only if on floor

    void Start()
     rb = GetComponent<Rigidbody>();

    // Update is called once per frame
    void Update()
         if (Input.GetKey(KeyCode.A))rb.AddForce(Vector3.left * forceMult);
         if (Input.GetKey(KeyCode.D))rb.AddForce(Vector3.right * forceMult);

         if (Input.GetKey(KeyCode.W))rb.AddForce(Vector3.up * forceMult / 2); 
         if (Input.GetKeyDown(KeyCode.W) && JumpReady == true)
            rb.AddForce(Vector3.up * forceMult * 24);
            JumpReady = false;
         if (Input.GetKey(KeyCode.S))rb.AddForce(Vector3.down * forceMult * 15);
        //Movement End
        if (Input.GetKeyUp(KeyCode.A))
            //Quick Stop A
        if (Input.GetKeyUp(KeyCode.D))
            //Quick Stop D

        rb.AddForce(Vector3.down * forceMult * GravityMult * 2);

Rigidbodys have a property velocity, see more about it here:

You can assing a new velocity, Vector3, with the x coordinate = 0:

rb.velocity = new Vector3(0f, rb.velocity.y, rb.velocity.z);

This should do the trick.

You can use rigidbody constraints in the inspector under rigidbody component. There just check " Freeze Position X"