AS2 help -- Disable dragging once movieclip reaches designated area?

Hi everyone!

I'm using Flash 8, AS2. To simplify things, I made an .fla using basic shapes, and have it attached here.

I have a black circle, that has a purple circle in it. I want to drag and drop the green circle onto the black circle, and have the purple circle immediately be sent to the yellow box.

My question is:

Once one of the colored circles is in the black circle, how can I disable the dragging of that circle? Make it immovable until the other colored circle takes its place?

Here's the actionscript I have already, which addresses everything EXCEPT disabling the mobility:

PHP Code:

---

on(press){
startDrag(_root.gbutton);

    dragging = true;
}

on (release,releaseOutside) {
stopDrag();
dragging = false;

        
if(this._x > 30 && this._x < 70 && this._y > 30 && this._y < 70)
        {
        setProperty(this,_x,58.0)
        setProperty(this,_y,54)
                
            dragging=false
}

if(_parent.pbutton._x == 58.0 && _parent.pbutton._y == 54)
        {
        setProperty(_parent.pbutton,_x,420.0)
        setProperty(_parent.pbutton,_y,40.0)
        }
        } 


---

And I have the same for if I want the purple to take the place of the green.

Please forgive any clunky script, and thank you so much for your time!