Edit in JSFiddle

document.addEvent('domready', function(){
     //创建一个 function object 
    var warning = function() {
        alert('You have clicked a link. This is your only warning');
        //警告后，并会删除网页上所有的<a>元素
        $$('a').removeEvent('click', warning);
    };
    
    //添加单击事件监听器，并执行 warning 方法
    $$('a').addEvent('click', warning);
});

<a href="#">Hyperlink 1</a> <a href="#">Hyperlink 2</a>