You have full control over your orders, you can cancel/suspend tickets at any point. Once a ticket is suspended it is no longer an active ticket and the event goer will be unable to check in to the event. You can reverse a ticket suspension at any point.
To suspend tickets
Go to your orders and select the order you wish to cancel/suspend
Click the Suspend button.
Enter a reason for the suspension for audit purposes and click Suspend Order.
To unsuspend tickets
Go to your orders and select the order
Click the Unsuspend button and confirm by clicking Unsuspend Order.
To refund individual tickets or full orders, follow this article.