You have to understand this is real human spam added in the forum. Despite reCaptcha being on or post throttling, they can still wait and post their crap.

To be honest, it's not Kunena's fault. Joomla! should offer "some" tools to prevent that.

Since that can't change anytime soon, we've decided to build our own solution - so we can make it public to others as well. This solution boils down to 2 things:
1. Prevent spammers from signing up in your site in the first place, with the use of massive spammer databases available for free (e.g. Bad Behaviour)
2. Prevent spammers who were not detected upon signup by using some basic text analysis before a comment or forum post in Kunena is added.

This will take about 2 weeks, so please bare with us until then.

I guess you've noticed by now that spam posts have been significantly reduced. We have integrated a third-party community-driven service and we are seriously considering adding this integration within K2. This service will basically identify spammers upon registration and block them.

We will follow up with a blog post and more details soon :)