super() with blocks -- how to avoid them bubbling up?

gistfile1.rb

#!/usr/bin/env ruby -w

class Foo
  def foo &block
    block.call "i am foo"
  end
end

class Bar < Foo
  def foo &block
    super()
    block.call "i am bar"
  end
end

Bar.new.foo { |s| puts "hello: #{s}" }

# outputs:
# hello: i am foo
# hello: i am bar

# how can you explicitly prevent the block from cascading up in the call to super?