gistfile1.rb

class Class

  def seal!
    class_eval do
      def self.method_added(method_name)
        Object.const_set(name, @sealed) if defined?(@sealed)
      end

      def self.singleton_method_added(method_name)
        Object.const_set(name, @sealed) if defined?(@sealed)
      end
    end
    @sealed = self.clone
  end
end

class Sealed

  def foo
    puts "denied"
  end

  def self.bar
    puts "denied again"
  end

  seal!
end

class Sealed

  def foo
    puts "wuh-oh"
  end

  def self.bar
    puts "double wuh-oh"
  end

end

Sealed.new.foo
Sealed.bar