class MyController < ApplicationController

  def my_action
    if current_user.nil?
      render layout: "new_user_layout"
    else
      render layout: "member_layout"
    end
  end

end

class ApplicationController < ActionController::Base
  layout :users_layout

  private
    def users_layout
      current_user.nil? ? "new_user_layout" : "member_layout"
    end

end

class MyController < ApplicationController

      def my_action
        #solo debes poner en modo false para  que lo desabilites solo para este metodo
         render layout: false
      end

    end

class MyController < ApplicationController
layout: false
  def my_action

  end

end