It's not possible to remove the "No Thanks" options from the dialog. Of course, you can modify the Appirater code to support such a thing. It wouldn't be a trivial task, but it wouldn't terribly difficult either.
On the large topic of requiring a user to rate your app before proceeding to the next level, even if they click 'rate app' there's no way to guarantee that they actually rated your app in the app store.