Normally, WSN's CAPTCHA option is sufficient to stop spammers from submitting to your site. It can be a bit annoying to the submitter, though, and hard to read at times. It can also be defeated by the more advanced or specifically-targeted bots.

Now you have another option: google's reCAPTCHA. reCAPTCHA has several advantages. First, it can be "solved" with just a click in most cases thus saving the submitter time and effort. Second, when it suspects the user of being a bot, it presents them with problems that are impossible for current AI systems -- such as choosing which of a series of pictures contain a certain thing. Third, the complexity is configurable so you can decide whether to make things easier or more secure.

The drawback of reCAPTCHA is that you have to sign up for keys for each domain name to use it. When you select it from the spam prevention menu at Admin -> Settings -> General, two boxes will immediately appear prompting you for the site key and secret key and there'll be a link above the boxes telling you where to register to get the keys.

reCAPTCHA is available in WSN versions 10.0.32 Beta 3 and later.