Disabling onclick attribute with javascript

How do I disable the other onclick event once one of them has been activated. Both divs are set to display:none; in the CSS. This is probably really simple but I am new to programming.

HTML

<a id="leftbutton" href="#" onclick="showDiv(this.id); return false;">click me</a>
<a id="righttbutton" href="#"onclick="showDiv(this.id); return false;">click me</a>

Javascript

function showDiv(id)
{   
  if(id == "leftbutton"){
     document.getElementById('orangediv').style.display = 'block';
     }else{
     document.getElementById('greendiv').style.display = 'block';
}}