Permanently, including everything attached to it.
Open RateMovies, go to My Ratings, and choose Delete account. You'll be asked to confirm. This happens immediately.
If you've already uninstalled the app, email ratemovies.app@gmail.com from the address you signed up with and ask for your account to be deleted. Requests are completed within 30 days.
Nothing is kept. Once your ratings are removed they no longer count toward the public Top 250, so the scores other people see will adjust accordingly.
This cannot be undone. If you sign up again later with the same email address, you start from an empty ladder.