The Kunena team has announce the arrival of Kunena 7.0.4 [K 7.0.4] in stable which is now available for download as a native Joomla extension for J! 5.4.x/6.0.x. This version addresses most of the issues that were discovered in K 6.2 / K 6.3 / K 6.4 and issues discovered during the last development stages of K 7.0
To be honest - I have not tried. The post definitely stay in the database. As for links, they will most likely point to never never land and will most likely put you back onto the homepage.
Our new class design for versions 1.1 and above will allow us to specify default behavior for such situations much better.
Until then a bit of manual cleanup might be required. One could remove the user-id for such posts and they would look like guest posts without a user profile link.