I have deleted the default admin account many times. First I needed to create a new admin account and then just login with the new admin and remove the old one. As ivanreis said please make sure to re-assign all the knowledge objects of admin to the new user otherwise they will become orphan and you won't be able to use them.
I never experienced this customer request before. You have the option to clone the admin account to a new account. Also is important to highlight that all knowledge objects already created under admin account have to reassigned to this new user.