Refund for 'out of stock' item- Order no ***

I bought shoes on the 5th of Feb 2018 from Zando. The order was placed and paid successfully except that the same afternoon I received an email from them that my order was cancelled due to unavailability of stock. They send me a voucher to the value of the money I paid, my problem is that they have failed me and they decide to give me a voucher as if everything is normal without my consent. I send an email same night for a refund since I don’t want anything to do with them, until now I haven’t received any response from them. Surprisingly they were quick to notify me about item out of stock and offer the voucher but they ignore my refund email. Their services is really bad and I wouldn’t recommend them to anyone else, they are money making scheme.