Test for comment spam, and automatically mark bogus comments as spam.
The blogspam plugin is a hands-free way of preventing the submission of spam comments. Once installed it checks each submitted comment against a centralised service and rejects SPAM automatically.
You will find your comment SPAM decrease significantly after enabling the plugin.
The plugin is free to install and use, and doesn't require registration. If you wish you can even run a server of your own to perform the testing!