You cannot disable the back button in a custom browser. This is a fundamental feature of browsers that cannot be overridden.
You can do this so that your application crashes (it displays an error message requiring the user to start working) if the user returns. This is a bad idea, because it is really a recognition that you did not accept the back button when developing the application. Each application, even order forms, shopping carts, online banking, etc. If they are correctly designed, should be able to use the back button. If they cannot, this is a failure on the part of the application developer.
One approach I've seen for intentionally violating the use of the back button is to pass a token to each application URL and within each form. The token is regenerated on each page, and as soon as the user loads a new page, tokens from previous pages become invalid. When the user loads the page, the page will only show if the correct token has been transferred (which was provided to all links / forms on the previous page).
The online banking application provided by my bank is as follows. If you use the back button at all, more links will not work and the page will not load anymore - instead you will see a notification that you cannot go back and you need to start all over again.
Now I see this as a lazy way to handle the problem. Online banking applications that do this will do this because it is theoretically easier to protect the application if they greatly limit the number of circumstances in which someone can arrive on a given page. This is, of course, a reduction compared to the fact that the application works properly as the user should expect, but those who develop online banking are less user-friendly and more focused on reducing potential attack vectors. They also probably also emphasized the need to deal with truly ancient computer systems and interfaces.