Hello @Vlad43,
Both your listings have instant book turned on so it 's perfectly possible for someone to send an inquiry first and then book instantly even though the inquiry is declined.
Of course it's not a good situation at the moment, but anyone who meets the requirements for instant book can book instantly without the host having to accept the booking.
You could contact Airbnb support to discuss the reason why you don't wish to accommodate the guests. They may be able to help with cancellation.
Type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.
Steve.