Hi,
In this thread from some months ago I included the JavaScript code to disable (hide) the "back" button.
However in that post I didn't actually explain the steps required as there was quite a few of them.
This is fine if you haven't modified your JavaScript file however in your case I think that you have as you mention that you have integrated it into a form.
So, before replacing your JS code with the code in that thread, remember to backup your current file with your modifications and then to copy them over to the modified version.
I'm afraid I can't help you out with the Wordpress side of things as I don't do much work with Wordpress.
Chris