stubbing CAPTCHA to pass Cucumber scenario

I'm trying to start using cucumber. I've done login, logout scenarious,
but stumbled in CAPTCHA validation on user registration page. As I get I
should stub CAPTCHA validation method. I guess I should implement
stubbing Given step or include stubbing code in Before filter?

I use validates_captcha gem, thus instance method to stub is

module ValidatesCaptcha
  module ModelValidation
    module InstanceMethods
      def validate_captcha
        # doing nothing means passing

I've read these posts
but still don't get where should I put stubbing code, should I call
stub! method and if so, of which object?