At Admin Panel -> Settings -> General, you have an option to select a spam prevention method. If you select "no protection" then spammers will be able to employ automated tools which fill out all the HTML forms on your site to send spam to you (and potentially to your submitters, if emailing of submitters/members is enabled).

If your server has GD enabled, you may use the CAPTCHA image. When someone makes their submission -- such as a link submission, member registration, reporting of a link, or emailing of a link -- they will be asked to copy the characters shown in the image. If they do not copy correctly, their submission will not be accepted. Most spam bots will be unable to read the image and so their submissions will be rejected. This method has a couple of drawbacks, though. Since it's visual, blind users employing screen readers will be unable to fill out forms. Also, because CAPTCHAs are so popular, spam bots are becoming increasingly good at reading them.

Note that only unregistered guests will have to solve the CAPTCHA -- it will not appear for members, since they've already solved one in order to register. Also note that the random characters in the image change only once per day, since they're a transformation based on a daily cron setting. Don't worry that the image shows the same characters for a while, this is to be expected and it will change the next day.

The other option is to use invisible field bait to trick the spam bots into exposing themselves so they can be blocked. When this is enabled, an extra field is placed in every form which is invisible to humans using modern web browsers. If the spam bot tries to fill out that field, the submission will be rejected.