Yes you can - providing that the vouchers have not yet been redeemed
Please go to the order history page of your account, identify the batch that contains the voucher/s then click on the red & white arrow at the end of the row.
You will then find options to edit individual vouchers or the entire batch at the right-hand side of the page as below:
Your Huggg account will be re-credited for any vouchers you cancel, as long as they have not been redeemed.
Please note: Recipients will not receive any notification that the voucher has been cancelled.