C# - Controlling Browser Forward / Back Functions In Web Application?

I'm writing a web-based application for internal use within the business where I work. It's a fairly complex application, with a lot of forms that will allow the user to view and enter data, which once saved will be stored in a database.

One thing I'm anxious to avoid is allowing a situation to exist where a user might enter large amounts of data in the browser, and then (either deliberately or inadvertently) navigate off the page without saving the changes. To this end, I have already implemented an entry page which opens up a new browser window in which there are no navigation controls at all; only what is provided on the web pages themselves.

However, there are two potential ways in which a user could still lose data:

The browser Close button is still enabled, and a user could potentially lose work by clicking it inadvertently. I can probably live with this, as it falls at the extreme end of helping the user not to shoot himself in the foot. In Internet Explorer (and, apparently, in Firefox) the Backspace button works like a Back button. I only discovered this accidentally, and have as yet been unable to find a simple way of stopping this behaviour. This is potentially a problem, as an inadvertent use of the Delete key (e.g. having positioned the cursor in a read-only textbox, or when the cursor isn't on any particular field in the page) will navigate off the page.

What I would like to do, as a minimum, is prevent Backspace from navigating off a page if that page has any user-writable fields on it and any of those fields have been changed by the user since the form was loaded. Ideally, I would like to disable this particular use of the Backspace key completely, while the user is logged into this web application. The two possible ways that I can think of, for achieving this, are: (1) clear the browser's history as each page is loaded, or (2) trap the Backspace key and only allow it to work if the cursor is positioned within a field whose text can be changed (e.g. a textbox).

how I could achieve either of these things? The solution needs to be programmatic, rather than something that has to be manually configured on every browser in the company.