django-registration is pretty extendable. One way to extend it is to provide a custom registration form. I'd recommend to use reCaptcha, e.g. with the widget and form field from here. Then it is as simple as writing a custom form class and registration backend (which is simpler than it sounds):
from registration.backends.default import DefaultBackend
from registration.forms import RegistrationForm
class RecaptchaRegistrationForm(RegistrationForm)
recaptcha = ReCaptchaField(label="I'm a human")
class RecaptchaRegistrationBackend(DefaultBackend):
def get_form_class(self, request):
return RecaptchaRegistrationForm
The last step is to tell django-registration to use your backend. That step is described in the docs (I couldn't find a HTML version of the docs, sorry)