the problem is that you duplicated them. to get rid of them you can't just say stop, you need to remove them. how? very easy. with the removeMovieClip(); code.
a way to solve this (and the only way that comes in my mind right now) is to add following code to the main snowflake:
if (_root.flakes == false){
this.removeMovieClip();
}
now you need to create a variable on the main timeline:
flakes = true;
and on the frame where the flakes shall stop falling:
flakes = false;
the problem is now: the main flake won't be removed because it's not been imported via actionscript or duplicated. this is why you should import the main flake with the attachMovieClip (); code.
how to do this? kinda long to explain, maybe you should take a look at some good tutorials.
i suggest the tutorial collabs.
w/e another way to solve this problem with the snow is:
make the flake variable as we did but don't remove the flake if the variable is flase. just make it invisible: this._visible = false;
i think that's it so far. i hope this helped, bye