All I can think of would be to use sessions, generate a random token (such as via [man]uniqid[/man]) and then add that to the URL in your JavaScript AJAX request. Then only return from the file called by the AJAX request if the $_GET value for that token matches the session value.
This would not be a 100% solution, as once the user accesses the main page, he could look at the HTML source and figure out how to request the page directly, but he'd only be able to do that for as long as the session remained active, meaning that he couldn't just bookmark it and access it directly whenever he wanted to.