A few anti-spam tools are used in BE. There are a couple of captchas such as Recaptcha and Simple Captcha.
Akismet filters and TypePad filters can also be turned on. There's also an extension for StopForumSpam that can be used.
You can also choose to moderate comments where you need to approve each one. There are also whitelists and blacklists in the BE control panel, where specific IP addresses or email addresses can be whitelisted/blacklisted.