How to implement an abstract class in Ruby

I know there is no concept of an abstract class in Ruby. But if it needs to be implemented, how do I go about it? I tried something like this:

class A
  def self.new
    raise 'Doh! You are trying to write Java in Ruby!'
  end
end

class B < A
  ...
  ...
end

But, when I try to instantiate B, it is internally going to call A.new which is going to raise the exception.

Also, modules cannot be instantiated, but they cannot be inherited too. Making the new method private will also not work.

Does anyone have any pointers?