@Bryan512 My suggestion would be if this booking is still within the 48 hr. free cancellation period for the guest, tell them as stated in your listing, your maximuum guest count is 9 and that no gatherings, parties, or extra guests or visitors are allowed and that they need to cancel asap to receive a refund so they can look for a place that suits their needs.
If they don't do that, contact Airbnb and tell them the guest has indicated they want to have up to 30 people in your 9 guest max listing for a party and that Airbnb needs to cancel this booking with no penalties to you.
Or if it was an Instant Book and you still have 3 free cancellations, you could just cancel it yourself, but you shouldn't have to use up those for such a blatantly not okay booking.
Absolutely do not allow this booking to proceed. These people are not to be trusted if they even entertained the idea that this would be acceptable.