I preassumed that your timer is working well and message is showed correctly.
Did you try to use jQuery? To disable submit button simply insert this line:
Is this what you wanted to achieve?
Also below I pasted you link with similar question resolved:
According to your code (and assuming it's working), do the following:
- Add jQuery to your site: http://jquery.com/
- Modify your CountBack function, adding code I provided after
document.getElementById("cntdwn").innerHTML = FinishMessage;and before