The bug is: In the "User Info Settings", there is a setting named "Enter names that should not be selected as username", but this function doesn't work.
How to fix it: Edit the file /register.php, go to line 83, and change "bad_emails" to "bad_unames".
(You may possibly worry about the Email verification, well, just set your heart at rest, the Email verification code is the lines begin from line 50.)