Inserting the following code will prevent users going back to the previous page -
In NS 7.1, Firefox 1.0 and IE 6, pressing the back button or selecting "back" from the right click menu will simply cause the present page to be redisplayed. History is not updated. I haven't tried it with earlier browsers.
I do not believe that you can get JS to display an alert when users hover over the back button, so you may need to warn them when they come to your site that you have effectively disabled the back button.
You haven't mentioned why you wish to disable the back button. If your site displays only virtual pages that are generated on the fly and displayed directly to the window, then you may also wish to disable the default actions of some of the function keys and Ctrl key short cuts (and this may not meet with the approval of some posters!)