Hey there! I get you, that's happened to me before. I believe it's because the person who use to have that username probably deleted their account, but what I still don't understand is why it keeps marking it as if it that username is currently being used.
When a person is banned from access, the history has to be kept for legal purposes. An account is never closed, merely deactivated. You have to pick a different name, that's all.
:(