@Siobhan103 These guests aren't entitled to a refund under the Guest Refund Policy, based on their failure to seek a remedy to the (non-)issues before leaving. But if they haven't yet completed the cancellation, their booking is still active. You might want to encourage them to go ahead and push that cancellation through, perhaps with the incentive of a partial refund for any dates you're able to successfully rebook. Once that cancellation is finished, you're not obliged to have any further contact with the guests.
With or without a refund, these guests sound likely to leave an unflattering review. I wouldn't worry too much about that, considering that it would be a complete outlier among many excellent reviews (including the positive one from S*****n in November, which you bafflingly wrote a negative response to). Next time, try not to shoot yourself in the foot with unnecessary negative comments - the Host Response should be addressed to prospective guests, not to the reviewer.
Also, when guests see a negative review, they can cross-check it against the reciprocal review you left for the guest to see whose story is more credible. That makes it especially important that you write an honest and straightforward review the first time around.