“R Antispam” is the only Joomla antispam extension for forums (Kunena, NinjaBoard and ccBoard) that uses a Bayesian algorithm. This means that spam recognition is improved over time.
It can be adapted to prevent spam in other extensions.
It works better if integrated with Akismet. You can get an akismet key at akismet.com. And if you think that you have to pay for a key, better look again and you will find that you can get it for free.
The extension adds a reference to the developer’s site, but you can remove this in configuration.