Not yourself, but you can contact Optus (LiveChat) and request they reset the log ins.
But unless you're getting locked out yourself you can probably just let your friend ride.
Just an update on this one.
The device registration limit was removed when stream kicking was introduced (If there are multiple streaming sessions, the first device will stop streaming after a few after the second device starts a streaming session.
You can now log into an infinite number of devices; however, the device cap remains.