A Metal app that looks for a timestamp on a form field, and makes sure it happened at least two seconds ago...

span_delay.rb

# Allow the metal piece to run in isolation
require(File.dirname(__FILE__) + "/../../config/environment") unless defined?(Rails)

class SpamDelay
  def self.call(env)
    @request = Rack::Request.new(env)

    if env["PATH_INFO"] =~ /^\/contact/
      if env['REQUEST_METHOD'] == 'POST'
        if @request.params['t'] && @request.params['t'].to_i < Time.now.gmtime.to_i - 2
          [404, {"Content-Type" => "text/html"}, ["Not Found"]]
        else
          [400, {"Content-Type" => "text/html"}, ["Bad Request"]]
        end
      else
        [404, {"Content-Type" => "text/html"}, ["Not Found"]]
      end
    else
      [404, {"Content-Type" => "text/html"}, ["Not Found"]]
    end
  end
end

Then change your form to:

form_tag contact_path(:t => Time.now.gmtime.to_i

or similar