First, make a backup of your website!
In older versions of Kunena, uninstalling via the Joomla Extension Manager only removes the dependencies from Joomla, so it is always possible to restore the forum by reinstalling it. This is no longer possible with the K7.x series (see Uninstall K7.x).
To completely remove Kunena, go to Kunena -> Tools -> Uninstall Kunena in the backend. After entering your password and confirming, Kunena and its associated database tables will be removed.