define touch area

First make a rect

 Rect notouchable_area;

Declare the size of rect in the Start() method.

     private void Start ()
         {
             #region Touch Handler (Touch will not work in these rectangles)
                      //Setting any of xMin, xMax, yMin and yMax will resize the rectangle
             notouchable_area = new Rect (getX (-50.2f), getY (-78.3f), getX (1100), getY (180));
             #endregion
         }
 public float  getX (float valX)
     {
         float x = (valX / 800) * 100;
         return  (x / 100) * Screen.width;
     }
 
     public float  getY (float  valY)
     {
         float y = (valY / 480) * 100;
         return    (y / 100) * Screen.height;
     }

Draw rectangle on the screen where you want that touch not work. You can see rect in the game mode using the onGUI method and change size according to your need.

 void OnGUI()
         {
          GUI.Box(notouchable_area,"");
         }

and in the OnMouseDrag method check whether touch exist in the rectangle area if yes then do nothing otherwise rotate or destroy object what you want.

Do this like i have done in the update method.

 void Update ()
     {
         if ((Input.GetMouseButtonDown (0)&& !notouchable_area.Contains (new Vector2 (Input.mousePosition.x, Screen.height - Input.mousePosition.y))))
         {
             // do whatever you want
             
         }
     }

If youa re not familiar then see the section https://docs.unity3d.com/ScriptReference/Rect-ctor.html

Edit:

I'm sorry I totally misunderstood the question but now this is what you want to do i think:

 using UnityEngine;
 using System.Collections;
 
 public class tes01 : MonoBehaviour
 {
     float rotSpeed = 8;
 
     void OnMouseDrag()
     {
             if (Input.mousePosition.y < Screen.height / 2) {
                 float rotX = Input.GetAxis ("Mouse X") * rotSpeed * Mathf.Deg2Rad;
                 transform.RotateAround (Vector2.up, -rotX);
             }
     }
 }

Still not sure if this is what you want to achieve but this should do the trick. In the OnMouseDrag() function you are checking if the mouse position on the y axis is lower than the screen height divided by 2 (lower part of the screen). @stephanlevin