Make 3rd Party booking obvious as a no-no to guests.
If a guest #1 inadvertently books for a partner guest #2 who does not have an account and then their partner guest #2 sets up an account and books the same dates (which their partner guest #1 cancelled for them) make it easier to refund the total paid including the AIrbnb fee to guest #1.
At present it is a nightmare of complex proportions to address this and if not addressed guest#1 ends up paying (as a family unit: guest#1 and guest #2) double the Airbnb fee. And if the cancellation is strict then a refund has to be issued by the host, which will be deducted from the next payment to the host even though the host has not received any payment.
Very complicated !