Advanced Instancing how to cleanup memory ?

Hello, i found how to render 1000 unit in documentation
panda3d.org/manual/index.php/Instancing

I have good performance with “Advanced Instancing”.

But i have question, how can i remove a one instance ?
In my code i generate 1000 human, how can i remove for exemple first human ? or 500 first human, or all ? for cleanup memory

from panda3d.core import *
from direct.actor.Actor import Actor
from direct.showbase.ShowBase import ShowBase
from direct.gui.OnscreenText import OnscreenText
import random

from camera import *

loadPrcFileData('', 'threading-model Cull/Draw')
loadPrcFileData("", "show-frame-rate-meter 1")
loadPrcFileData('', 'sync-video False')


class MyApp(ShowBase):
    def __init__(self):
        ShowBase.__init__(self)

        #custom camera driver, not important for the demo
        base.disable_mouse()
        self.cc=CameraControler(pos=(0,0,50.0), offset=(0, 250, 250), speed=1.0, zoom_speed=10.0)

        #add some light
        spot = Spotlight("spot")
        spot.set_color((1.0, 1.0, 0.85, 1))
        spot.set_exponent(16)
        spot.get_lens().set_near_far(5, 400)
        spot.get_lens().set_fov(70)
        #spot.show_frustum()
        spot_path = render.attach_new_node(spot)
        spot_path.set_pos(-80, -80, 180)
        spot_path.look_at(Point3(20, 30, 40))
        render.set_shader_input('spot',spot_path)

            
        #Advanced Instancing
        dancer = Actor("gpu_rocket.bam", {"kick":"a_rocket_walk1.egg"})
        dancer.loop("kick")
        dancer.setPos(0,0,0)
        chorusline = NodePath('chorusline')
        for i in range(1000):
            placeholder = chorusline.attachNewNode("Dancer-Placeholder")
            placeholder.setPos(i*5,0,0)
            dancer.instanceTo(placeholder)
            
        placeholder = render.attachNewNode("Line-Placeholder")
        placeholder.setPos(0,10,0)
        chorusline.instanceTo(placeholder)
            
            
        #Remove first instance ???

app = MyApp()
app.run()

i have second question, what is difference if i enable or disable this parameter :
loadPrcFileData(’’, ‘threading-model Cull/Draw’)
i not observe multicore in my cpu (I5 with 4 core) ?