Either way, it seems pretty easy to implement. Just checking against the referrer and returning to the homepage if it was found to be elsewhere should be sufficient.
Something like this maybe:
if($_SERVER['HTTP_REFERER'] != 'http://yoursitewhatever.com'){
header('Location: /');
}
Once the form has submitted (~line 1138 on index.php).
I'm not that good with PHP, but I think that
header('Location: /');
won't end the script. So it will send the coins either way and only redirect to main page after that.
header('Location: /'); die();
should work though.