If you haven't yet understood why, let me clarify...
The K2 Community is a very popular site. It's got an Alexa rank of about 10.000 and a 7/10 Google Pagerank. It does a little over 1 million page views per month on average. Spammers love such facts. Not spam bots, actual human spammers.
The past 2 weeks we are being heavily spammed by real human beings with regular Joomla!, Twitter or Facebook accounts (you need a mobile number to get a FB account and bots don't have mobiles). Not mention this has happened again (back when we hosted the community site on the Ning platform which is supposed to have a much better platform to fight spam)... Guess when? When effing Iron Man 2 was released. Spammers love blockbusters.
So what now? We're temporarily disabling new user signups for a few days in order to harden security measures on the site. That includes social logins as well. Fighting real human spam is a very tough situation to deal with. That means we need to add more services like Akismet and extend the integration with StopForumSpam.com to better fight these spammers that have been identified in these huge spammer databases. We also need to improve Kunena's code to allow for quick spam user banning. And since the version of Kunena we're using won't be updated anymore, we're going to get our hands dirty :)
So please bear with us for a few days until we sort the matter. It's a tough call, but I guess it's the price to pay for a popular project.
P.S. If you need to ask a support question and you are not already a K2 Community member, please use Twitter if possible to direct your questions to @k2devs - we'll do our best to answer you there.
Update on May 15th, 2013: We have re-enabled signups to the community.