One reader has pointed that although requiring a user to register is a good idea, some bots are able to do so, and has sent some "tricks" that administrators should use to prevent bots from registering in the forums.
First you should use security plugins. We have gotten accustomed to see them when applying for a new email account. An obfuscated key is shown to the user. You need to type this key to proceed. This is quite useful.[Imageattachment]
As an example, there is also a mod called "The Humanizer" which simply adds another question to the registration process. In particular "Are you a human being?" and two possible answers yes or no. Although this may sound quite obvious, it works. A general purpose bot, will not be prepared for these small modifications.
So the lesson is that there is no need for complex solutions, but smart ones. Making small changes once in a while can bring benefits. It is also advisable to spend a couple of hours performing these administrative tasks, instead of wasting time deleting spam messages
Thanks to David San Jose for this information.