Well, I think you have a couple of options.
You could track down the file that S2 uses to create that file. I don’t know which it is. But if you change that file then update your version at a later time, you will have to re-do it again if it gets replaced by a newer version.
The second option is to hook into the paypal return hook and re-direct to your own page.
That can be done like this:
<?php
add_action('ws_plugin__s2member_during_paypal_return_during_subscr_signup_w_update_vars','my_pp_return');
add_action('ws_plugin__s2member_during_paypal_return_during_subscr_signup_wo_update_vars','my_pp_return');
function my_pp_return(){
//do any processing if you want...
$thankyou_page = site_url('replace-with-your-thankyou-page-slug');
wp_redirect($thankyou_page);
}
?>
In case you don’t know, that info should go in a new (or existing) file here: yourwebsite.com/wp-content/mu-plugins/s2hacks.php
There are other hooks that you could possibly user for paypal return that can be found here:
http://www.s2member.com/codex/stable/s2member/paypal/package-filters/
Hope that helps,
Dave
PS. I’ve not done extensive testing but it should work. :)