How do I give a full refund?

I , as with many others, have received many cancellations and am refunding people in full if they do not wish to take credit.  I have just received a cancellation for May, outside the March to end of April guideline but have promised the guest a full refund.  However, booking.com will only allow me to give them 50% .  How do I refund the other 50%?  There seems no way on the booking.com site.

From now on, however, although it is obvious that nobody shall be travelling to Spain in May either, I shall no longer take cancellations until further notice from people booked 1st May onwards.

Anyway, need some advice as to how to refund the present cancellation the rest of their money.

Thanks