The back button belongs to the user, NOT TO YOU. It is not yours to disable.
I am getting tired of people who want to own take over other people's computers for their own selfish purposes (usually sales or advertising). Right now, I can't see the text insertion cursor, because some ad company has a picture of Saturn with moving moons on the screen.
And I am particularly tired of people who want to keep me from returning to the page I came to their site from. I have several times taken a side chain during a search, only to find out that my main search was destroyed by some clown erasing my history.
It makes people very mad at you.
why oh why when someone asks to do something does some prat always talk about Internet ethics like everyone is developing a public site - Business Application development is very different and can require custom functionality or restrictions to meet the CLIENT needs. What makes people mad is moronic statements from illinformed novice developers spewing regurgitated generic nonsense.
An alternate approach, in sessions its not so difficult, is to make the page function differently if [back]ed into(also if direct accessed when you dont want it).
the code is already in the page so no need to mess with browser history, but it displays just a "can't do this , Return to correct page, or, Login/Logout"