Raycast Mouse Click On Specific Objects Only

I'm trying to place cube primitives on a surface I've tagged as "Floor" When one clicks on the floor, a cube is placed at the mouse coordinates.

My trouble is that I'm trying to limit the cube placement to occur only when the "Floor" is clicked.

I want the function to ignore the cubes... That is don't place a cube on top of another cube.

Any suggestions on how exclude the cube's collider?

Here is my most recent attempt, which doesn't work.

 public class GetMouseCoordinates : MonoBehaviour
     {
     
         public Material theBlockMaterial;
     
         void Update()
         {
             CastBlock();
         }
     
         private void CastBlock()
         {
             if (Input.GetMouseButtonDown(0))
             {
                 RaycastHit hit;
                 Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
                 if (collider.Raycast(ray, out hit, 1000.000f))
                 {
                     if (hit.collider.tag == "Floor")
                     {
                         GameObject cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
                         cube.transform.position = new Vector3(Mathf.Floor(hit.point.x), Mathf.Floor(hit.point.y) + 0.5f, Mathf.Floor(hit.point.z));
                         cube.renderer.material = theBlockMaterial;
                         cube.transform.parent = this.transform;
                   
                     }
                 }
             }
         }
     }