The following are names you might want to prevent form being used not only by users, but also anyone that uses the admin to your site.
about, aboutus, admin, administer, administor, administrater, administrator, anonymous, auther, author, blogger, contact, contactus, contributer, contributor, cpanel, delete, directer, director, editer, editor, email, emailus, guest, info, loggedin, loggedout, login, logout, moderater, moderator, mysql, nobody, operater, operator, oracle, owner, postmaster, president, registar, register, registrar, root, signout, test, user, vicepresident, webmaster
A few of them are misspelled, typos, common login names, names hackers usually try to use to break in, and other names that regular users probably shouldn’t use as they would pretend to be someone they are not. Of course you may want to add the ability to prevent any starting with or ending with admin or moderator.
This is a good list to start with, share any others if you like. I didn’t include any cusswords, but it’s a good idea to add those into another table and make sure people don’t use those for usernames or just for part of it. Also this list took into consideration that the shortest username would be 4 characters long and only allowed letters and numbers.