class A
    def initialize(options = {})
        #what goes on in here doesn't really matter for the purpose of this question
        #I just don't want it to be initialized directly
        options.each do |k,v|
            instance_variable_set("@#{k}",v) unless v.nil?
        end
    end
end

class B < A
    attr_accessor :class_specific_opt
    def initialize( options = {} )
        @class_specific_opt = "val" unless options[:class_specific_opt].nil?
        super(options)
    end
end

class A
  def self.initialize_allowed?
    raise "You cannot initialize instances of this class."
  end

  def self.allow_initialize
    class << self
      def initialize_allowed?
        true
      end
    end
  end

  def initialize(options = {})
    self.class.initialize_allowed?
    options.each do |k,v|
      instance_variable_set("@#{k}", v)
    end
  end
end

class B < A
  allow_initialize
end

B.new #=> #<B:0x00000102837d10>

class <<A
  undef :new
end

A.new #=> NoMethodError: undefined method `new' for A:Class

private_class_method :new

public_class_method :new

class A
  private_class_method :new
end

class B < A
  public_class_method :new
end