There's no check for this. I just tried. You can delete a username and reregister it.
The only thing that might be confusing you.... if you turned email verification on, then the new username won't show up under admin (and you can't reregister or login) until you click the link to activate the account.
You can do a manual query to see what you have in the database:
SELECT U_Approved
FROM w3t_Users
WHERE U_Username = 'xxxxx'
That'll tell you the user's status. And if you need to, you can delete:
DELETE FROM w3t_Users
WHERE U_Username = 'xxxxx'
But you should be able to reregister a deleted user, as long as it's not banned or on the "bad names" list.
