Ditto what Coyote said. Don't try to control the user's behavior. The back button is the second most used control (after clicking a link) on the web.

I'm sure you have a good reason for not wanting the user to click back, but you should probably change your design to allow for it. One of the nice things about the web is that you know you can follow a link and always get back to where you are. If you keep then from going "back" to your page, you keep them from going "back" anywhere. What then? Make them close the browser?