Professional :: No More Clicks Allowed When 15 Displayed In TextBox

I am currently working on a simple program that when the user clicks a button it adds 1 to a textbox for whatever button has been pressed. For example they click a heads button it puts 1 into the box, they click 1 in the tails button and it displays another 1 in the the box. What I am having a problem with is that once the total number of clicks has been done (15) I want it to stop and not allow anymore clicks.
 
Here is my code so far:
btn_Heads.addEventListener(MouseEvent.CLICK, runHeads);
btn_Tails.addEventListener(MouseEvent.CLICK, runTails);
btn_Clear.addEventListener(MouseEvent.CLICK, clearData);
var coinArray:Array = new Array();
var headsTotal:int=0;var tailsTotal:int=0;
var counter:int=0;
[Code] .....
 
The code runs all and well but when the total value reaches 15 I want it to stop and not allow anymore clicks.I was trying a for loop. Basically all I need it to do is that the Box's that are getting numbers put in them I need the total 1 to stop at 15 and not allow anymore changes.