Code for blog post "Simple Rails Form Honeypot" - http://nomethoderror.com/blog/2014/02/26/simple-rails-form-honeypot/

1_form.erb

<%= form_tag form_submit_path do %>
  <fieldset>
    <legend>Contact Info</legend>

    <div class="form-group">
      <%= label_tag :name %>
      <%= text_field_tag :name %>
    </div>

    <div class="form-group">
      <%= label_tag :email %>
      <%= text_field_tag :email %>
    </div>

    <div class="form-group">
      <%= label_tag :phone %>
      <%= text_field_tag :phone %>
    </div>

    <div class="form-group honeypot">
      <%= label_tag :content %>
      <%= text_field_tag :content, nil, tabindex: '-1' %>
      <span class="help">Do not fill in this field. It is an anti-spam measure.</span>
    </div>
  </fieldset>

  <button type="submit">Submit</button>
<%- end -%>

2_styles.scss

.form-group {
  margin-bottom: 1em;
  width: 100%;

  &.honeypot {
    position: absolute;
    left: -9999px;
  }
}

3_controller.rb

class FormController < ApplicationController

  def submit
    SiteMailer.notify(params).deliver unless params[:content].present? # honeypot check
    flash[:success] = "Your message has been submitted. Thank you!"
    redirect_to root_url
  end

end