delete this.onPress

I am making a cool puzzle game where you can do heaps of secret little things to get points or to go to another part of the level.

I made a movieclip that everytime you click on it plays until the next stop();
it all works great except one thing...

this.onPress = function(){
play();
}

this is what i use to make the movieclip play. i do not want this changed.

i want to make it so that on the last frame it will delete the this.onPress so i tried using delete this.onPress;

but i click on it again while it is on the last frame and it goes back to the first!!! how can i delete the onPress?

can anybody tell me how to get rid of this.onPress

on last frame just set this.onPress=function(){}; or this.enabled=false;

ok cool