How to disable A, S, and D keys for third person player movement

Hello! I'm new to scripting and I have a quick question. On my third person game, I don't want players moving left/right or down. They can only move forward while using the mouse to change directions. The script I have now uses WASD keys to move. How do I make it so that the A, S, and D keys don't do anything when pressed? I searched all over for the answer but can't seem to find anyone who has this scenario. Here it my player movement script:

     private void CheckInput()
     {
         float vertical = Input.GetAxis("Vertical");
         float horizontal = Input.GetAxis("Horizontal");
         float horizontalrotation = (Input.GetAxis("Mouse X") * Time.deltaTime * 100);
         float rotateSpeed = 28f;
         transform.position += transform.forward * (vertical * moveSpeed * Time.deltaTime);
         transform.position += transform.right * (horizontal * moveSpeed * Time.deltaTime);
         transform.Rotate(new Vector3(0, horizontalrotation * rotateSpeed * Time.deltaTime, 0));