The answer I finally found is to only display the form elements that you want:
<form action="/app_dev.php/SignUp/create" method="post" name="registration">
<div id="registration">
<div>
<div id="registration_user">
<div>
{{ form_label(form.user.email) }}
{{ form_widget(form.user.email) }}
</div>
<div>
{{ form_label(form.user.username) }}
{{ form_widget(form.user.username) }}
</div>
<div>
{{ form_label(form.user.password.password) }}
{{ form_widget(form.user.password.password) }}
</div>
</div>
<div>
{{ form_label(form.terms) }}
{{ form_widget(form.terms) }}
</div>
<div>
<button name="registration[Sign Up]" id="registration_Sign Up" type="submit">Sign up</button>
</div>
{{ form_widget(form._token) }}
</div>
</form>
This is documented in the Symfony forms chapter.