As for as i understand u cant able to control the browser back button.
That is u can able to provide alert message when back button is clicked and u cant able to disable the browser back button.
I can suggest u can idea.
Let me say u are using Session["UserID"]. When the user logged in the session["UserID"] is assigned with the value and it the user logged out session["UserID"] is set to null.
In every page, just validate whether session["UserID"] is null or not.
If it is null means redirect to the login page or any page as u wished.
So when the user press the log out button in the master page, the sessio["UserID"] is set to null.
When the browser back button is clicked, the browser will move to the previous page.
when the user hit any action in that page, during the page load it checks for the session["UserID"] and at that stage it is null and it is redirected to the login page.
If you see any ticket booking site or any net banking site, during the transaction they used to show message "Please dont press back button".
From that u can clearly understand that browser button cant be controlled.
So try my idea and let me know if u are facing any problem.
Thanks.