If your account is compromised, the system will slam the doors closed on your games and send you an email with all of the details. The email also contains a one-time unlock code. This code allows players to open their accounts in the event of a mistaken intrusion.
This system sounds very similar to Trion Worlds' coin lock mechanism. Players have a choice whether or not to participate in Account Guard, as they can opt out of the service through the account page. The studio has posted a FAQ with more answers to particular scenarios.