How To Disable The "Back" Button?

Yes, I would like thoughts on how to intentionally commit web usability sin and "break the back button" but please don't freak out on me yet. I've had quite a few years of experience in web development at this point and I understand the need for usability. This is precisely why I need to disable the back button, or at least make people need to think before they use it. Its a service to the end user in this case & an attempt to prevent them from making a mistake.

I am currently working on a web application that is almost entirely AJAX. (Lets save the "is AJAX worth the hype" discussion for another thread. It has its place & I'm not using it trivially.) It is designed in such a way that it is intended to function as a desktop application, and, other than the fact that it is on the web & rendered in HTML/JS, it really bears no resemblance to a traditional "web site". There aren't "pages" and a "history", there is just a user interface used to manipulate the data and update the view. All the different views occur on a single page, just as any single window desktop application would.

Now the problem: Because its being developed to run in a browser, people will still have the "web site" mentality, not understanding the huge difference between "web application" and "web site". If they do open a new view and then want to go back to what they were previously looking at, they will want to hit the back button, but in the context of this app, that may seriously goof them up, even resulting in loss of work.

Example: Lets say that a user opens "data_entry.asp" and does some work. They then fill out a form but need to look up some information. They click a "Look Up" sort of button to get the information they need. When they click it, the data entry form is hidden (note that their data entry is unsaved, but perfectly safe since it is still on the page) & replaced with the info search form which uses AJAX to perform their search. Now to get back to their form, they SHOULD click the "close" button, which just re-displays the data entry form, but they've been conditioned to hit "back". When they do this they will go back to the page they were on before, resulting in the loss of everything they entered on the form.

So what I need to do at least is alert them hen they click the back button "Hey, you're about to leave this page, but there's really no reason to do that unless you're really done." & give them the chance to back out of using the back button (no pun intended).

Anyone have any idea how to disable the back button, or have any idea how to tackle the problem at hand. I don't want to revert to using separate pages and having to store temporary data to re-populate forms. In this kind of app thats a lot of extra & unnecessary work.