@Sam4179 In my opinion... It depends
Is it legal for you to rent to this guest for those dates? If not then I believe you should refund in full
What is Spanish law with respect to contracts that are illegal to fulfil?
Finally a fair approach, if you are legally entitled to maintain your strict cancellation policy, is to ask the guest to cancel ASAP and then tell them you will refund him to the extent you are able to re-let the space.
Alternatively the guest will tell Airbnb he has Corona Virus and they will be fully refunded!