@Kim-And-Adam0 Is this an Instant Book guest that hasn't responded to any of your correspondence? If so, Airbnb might grant you a penalty-free cancellation. But you would have to call Airbnb to confirm this; what you definitely should not do is manually cancel the booking yourself.
Your suspicions may well have some merit. Is the guest based in your local area? Is the booking for only 1 or 2 nights? Are two cars more than you'd expect from a group of their size?
Your listing does not appear to have a rule about unregistered guests/visitors, and merely saying "No Parties or Events" does not account for the wide range of opinions on what does and doesn't constitute a party. This will make things a little harder for you. But you can still contact the guest to clarify that the number of booked guests are the only people that are permitted on the property at any point in the stay, and if the guest fails to give you assurance on this point, you'll be on firmer ground to pursue an administrative cancellation. For future bookings, though, I strongly suggest updating your House Rules to reflect in specific language what you are trying to prevent, so that you need not make a distinction between younger and older guests that some might find discriminatory.
You can also take precautions to prevent parties. External cameras and noise monitors are the choice of some hosts, though a longer minimum stay and no instant-book are in my opinion the most effective filters.
