Issue with usernames when registering

When new users register, it doesn't let them sign up with their desired username. They choose an alternate one, then once the account is activated, I (admin) have to go in and change it to what it should be. This problem occurred ever since the forum was created, and it's starting to get annoying.
http://intotheoblivion.forumotion.com

hello lets try something go to
ACP (Admin panel)-->General-->Security--> 'Dis allow subscriptions with the most commonly used spam usernames?' , tick 'No' and save.