This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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.