Ignore collisions between objects

Okay, so i have these robots, big robots, that i want to kill people when it gets near them, so i have a sphere that follows their feet. and it is scripted with the following instadeathcollider:

function OnTriggerEnter (col : Collider) {
    var player : FPSPlayer = col.GetComponent(FPSPlayer);

    if (player) {
        player.ApplyDamage(10000);
    } else if (col.rigidbody) { 
        Destroy(col.rigidbody.gameObject);
    } else {
        Destroy(col.gameObject);
    }
}

function Reset () {
    if (collider == null)   
        gameObject.AddComponent(BoxCollider);
    collider.isTrigger = true;
}

How do i make it so it ignores the collision between the sphere and the robot ???? Physics.IgnoreCollision?

You could set up your robots with tags and then query whether or not the object you're colliding with has your robot tag. If it does, you early out.

http://unity3d.com/support/documentation/Components/Tags.html

Like this

function OnCollisionEnter(collision:Collision)
{
    if(collision.gameObject.tag == "Robot")
    {
        Debug.Log("it's a robot");
        return;
    }

    // rest of your code
}

Yes. Physics.IgnoreCollision is exactly how you'd ignore physics collisions between two objects.