@Sabyasachi1
Both reservations are "long term stays", having the "long term stay cancellation policy" attached.
Both reservations are not providing a refund for first 30 nights. So technically the guest is entitled to receive a refund for 14 nights on the second reservation, if cancelled before check-in.
It is the guest's responsiblilty to cancel a reservation, if technical issues the guest can contact Airbnb.
You have no obligation to refund more then the cancellation policy calculates.
And if you decide to provide more refund, you can do it yourself via resolution centre, after the reservation is cancelled by the guest or Airbnb.
You can report abusing messages to Airbnb via the "flag" link on the message, also provding the option to block the guest (recommended after case is closed)