Yes you can - providing that the vouchers have not yet been redeemed.
Here's how to cancel an entire order:
Head over to your 'Voucher History' page (left hand tab).
Find the relevant order, tick the box next to it and you'll see an option to 'Cancel'.
If you're 100% certain, please Hit 'Confirm Cancellation'.
Here's how to cancel a specific voucher:
1. Head over to your 'Orders' page.
2. Locate and click into the relevant order.
3. Find the Huggg link that you'd like to cancel and click on the rubbish bin icon.
If you're sure, click 'Confirm cancellation'.