there really is nothing you can do about this if they book with someone else. if they book with you under another listing you have, you can cancel them right away stating there is a covid concern. just keep an eye out for this guest in the future when they book again with you. Good Luck!
My concern is that airbnb should prevent people who present covid +results to travel to other airbnb listings until they present a -resulta. I feel is airbnbs duty to at least inform the other host.
As soon as she was refund, the virus disapeared and she felt much better to travel. Airbnb is the best doctor ever!
More seriously, it is nonsense that people who cancel for 'illness' reasons can book again on the same dates. They should be blocked as hosts have the days closed in their calendar.