This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread 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.