This week's book giveaways are in the Refactoring and Agile forums. We're giving away four copies each of Re-engineering Legacy Software and Docker in Action and have the authors on-line! See this thread and this one for details.
You can't. This is a good thing. The back button isn't yours. It belongs to the user of the browser and is there for him/her to navigate across different sites as well as different pages within a single site.
It's not for the developer of any particular site or application to decide whether or not this feature is available to an end user.