How do I de-activate a button after it's clicked once

I'm doing a simple quiz and I only want the user to add to the total score once if he presses the correct button. How do I stop him from being able to click it multiple times using script rather than just adding a key frame and removing it's instance name or replacing the symbol altogether. This is what I have on the frame for the button.

car_btn.onRelease = function() {
right = new Sound();
right.attachSound("right");
right.start(0,1);
subTotal++;
gotoAndPlay("correct4.1c");
};