Mollom is an excellent way to stop spam in your website. I am using mollom on this website and I have saved myself a lot of administrative time by having spam blocked by it. It is super easy to setup you just need to open an account with mollom and setup it up (drupal module for me). If mollom is unsure a comment is spam it presents a captcha, and there you go, no spam and no necessary inconvenience to every legit user.