A simple plugin to provide honeypots and the necessary validations. It add fields to your forms, empty and filled, then checks if there values are unchanged.

script/plugin install git://github.com/grosser/honeypot.git

Controller:

before_filter :check_honeypots, :only => [:create]

OR - if the default render new + flash[:error] is not what you want

def create
  render :action=>'error' and return unless honeypot_untouched?
  ...
end

Inside form:

=honeypot

If you want more sophisticated protection, (on cost of maintainability and usability) try the negative_captcha plugin