Please have a look at my answer to a similar question earlierHow to disable back,forward and Refresh functionality in browser?
In short, don't disable the back button - it's not yours to disable. If you're trying to do this because your application 'needs it', then your architecture and general approach is probably incorrect
"..But i have to disable it in every page of my project. How can i do this?"
Why would you ever need to do this? What about if you were on codeproject now and you couldn't use back or any navigation functions, wouldn't you be annoyed?!
If you really want to stop users being able to go back, you could use an approach similar to this linkhttp://www.codeguru.com/csharp/.net/net_debugging/debugging/article.php/c12891