Press the blue button to instantiate cubes, press the red button to destroy them.

the_button_giveth_life_and_the_button_taketh_away.cs

// Game object "index" refers to the LeapMotion index finger
private void OnTriggerEnter(Collider c)
{
    if (this.name == "blue")
    {
        if (c.gameObject.transform.parent.name.Equals("index") && numPrefabs < MAX_NUM_PREFABS)
        {
            prefabs[numPrefabs] = Instantiate(prefab) as GameObject;
            numPrefabs += 1;
        }
    }

    if (this.name == "red")
    {
        for (int i = 0; i < MAX_NUM_PREFABS; i++)
        {
            Destroy(prefabs[i]);
        }
        numPrefabs = 0;
    }
}