Toggle button using the ! logical not operator

I’m on page 583 of the best ActionScripting book I’ve ever seen (and I have 5 or 6 of them): Flash MX AS “Training from the source” by Franklin and Makar.

I’ve done every tutorial exercise in this book–no problem, … til now.
On page 583 they have a button to turn off (actually toggle) a multi-banner-loading movie and they coded it like this:

on(release) {
_level1._visible = !_level1._visible;
bannerBack._visible = !bannerBack._visible;
}

If I click the button while the banners are loading, the present one will stop but then, after a slight delay, the sequence starts up again of it’s own accord.

Am I missing something or is this failing? The authors imply it should stop the banners from showing (if they’re currently loading) because “it’s set up to control the movie loaded into Level 1 rather than a particular banner movie.”

I tried to get Flash Help to help me but it’s only sample for this “logical not operator” goes like this:

var happy:Boolean = false;
if (!happy) {
trace(“don’t worry, be happy”); //traces don’t worry, be happy
}

and I’m not hip enough to see how it might aid in changing or understanding the book’s code.

I tried searching this site and the web every which way but can’t find any code to create a toggle button with the “not operator” that uses the explanation point.

Any ideas as to what’s going on and what can or should be done in such a situation?