Objects will not destroy after they have been instantiated

@jkish1990, this damned UA text editor seems to have eaten part of your code, in the j loop. I don't know what exactly it was before, but tried to calculate xpos in some logical manner, and selected the prefab randomly based on a 50%/50% cube/sphere ratio.
Anyway, you were destroying gameObject, which's the reference to the object to which this script is attached - you should instead destroy the objects you have created, but this is impossible at DestroyClones because you don't have references to the objects! Fortunately, there's a simple solution in this case: call Destroy(clone, randomtime) right after creating an object (where clone is a reference to the object created) - this will kill the object after a randomtime delay. Since all objects are created in the same frame, they will also die in a single frame, randomtime seconds after - just wait the randomtime delay and clear the variable isSpawned:

var grid: int = 4; //size of grid var isSpawned : boolean = false; var spawncubePrefab : GameObject; //cube var spawnspherePrefab : GameObject; //sphere

function Update () { if (isSpawned == true){ return; } else { CreateGrid(); isSpawned = true; } }

function CreateGrid() { var randomtime = Random.Range(3,7); // generate the random life time for (var i=1; i<(grid+1); i++){ // create the objects var zpos= (2.5*((i*i)+i+4)); //creates rows at certain distance for (var j=0; j < grid; j++){ var xpos = 2.5 * j; //creates columns at certain distance var prefab: GameObject; if (Random.value >= 0.5) { // select the prefab randomly prefab = spawncubePrefab; } else { prefab = spawnspherePrefab; } // create the object and get a reference to it: var clone: GameObject = Instantiate(prefab, Vector3(xpos,2,zpos), transform.rotation); // program the object destruction time: Destroy(clone, randomtime); } } yield WaitForSeconds(randomtime); // wait randomtime... isSpawned = false; // then clear isSpawned, since all objects have suicided by now }