Put this at the end of a Vagrantfile that uses the chef-client provisioner

vagrantfile.rb

module Vagrant
  module Provisioners
    class ChefClient < Chef
      def cleanup
        if env[:vm].config.vm.host_name
          puts `sh -c 'knife client delete #{env[:vm].config.vm.host_name} -y'`
          puts `sh -c 'knife node delete #{env[:vm].config.vm.host_name} -y'`
        else
          puts "No host_name was defined for the box... unable to remove it from chef"
        end
      end
    end
  end
end