Transactions can be cancelled by selecting the "View All" option located next to the "Recent Transaction" feature found in the dashboard of your account. You'll then see all of your recent transactions and should have the ability to cancel your transaction.
Please note, transactions can only be cancelled before 5pm on the day they are initiated (or the next day before 5pm if the bonds were bought after 5pm). If you miss the cancellation deadline, because our bonds have no sale restrictions, you can simply sell the bonds as soon as the transaction clears to quickly return your funds.