You have the ability to void tickets, making them unusable, right from the transactions page.
If you don't know the specific event, go to the general 'Transactions' page on your dashboard pop-out menu.
However, if you do know the event, you can go to the event specific 'Transactions' page.
On the left hand side, you will have multiple options to help you filter and find the transaction that you are looking for.
Once the transactions is found, select the gear box in the bottom left hand corner and choose 'Void.'
Please note: A ticket that is void does not mean that it has been refunded.