when the customer gets to the paypal site, the order is not placed until they click the submit order button on the paypal side. If they use the browsers back button the order will never be placed with paypal.
they have to click the submit order button for paypal to begin processing the order and send them to the success page.
there really not much that can be done to prevent them from using the back button, but if they do, they wont process the order.