It all depends on which platform you are using for your website. If it is a self hosted wordpress then spam regulating plugins like akismet can do the work. For a free wordpress site or a blogspot blog on blogger the platform itself will manage the spam control for you. If you have your own system then you may control all incoming comments with website links to moderation. Website names can also be included in the radar. Above all of these a reCaptcha module from Google can stop automatic spam bots to a very extent. If you have a dynamic website like forums then you can ask other members to fight against spam like answeree.com does.