禁用的f12怎样起用

/*
keyCod从113-123分别禁用了F2-F12键,13禁用了回车键(Enter),8禁用了退格键(BackSpace)
jamlu制作
MSN:javailoveyou@hotmail.com
*/
var str="系统消息!!!!这个键已经被系统禁用了!";
document.onkeydown = function(){
if(window.event && window.event.keyCode == 113) {
window.event.keyCode = 505;
}
if(window.event && window.event.keyCode == 114) {
window.event.keyCode = 505;
}
if(window.event && window.event.keyCode == 115) {
window.event.keyCode = 505;
}
if(window.event && window.event.keyCode == 116) {
window.event.keyCode = 505;
}
if(window.event && window.event.keyCode == 117) {
window.event.keyCode = 505;
}
if(window.event && window.event.k