@Ellen912
Do you have Instant Book turned on? Please double check your settings as twice now, it had turned back on after I turned it off.
If you chose to use IB, then you can cancel 3x a year penalty free (after that you need to ask CS to do it) if you are uncomfortable with the guest or feel they are going to break your house rules. A 1.5* review is a good enough reason to feel uncomfortable about the guest in my opinion. What did the review actually say was the problem?
Also, if you want to use IB, you can select certain filters like 'must be recommended by other hosts', verified ID and a response to your pre-booking message (although the latter is pretty pointless as it's a post-booking, not pre-booking, message in reality). You can require a profile photo for IB or request bookings, but you'll only see this after it's confirmed.
If you are sure you are not using IB then no, this definitely should not have happened, but I had a weird situation yesterday and I am wondering if this is the same glitch, i.e. a request booking going through even though I never received the request, let alone accepted it:
https://community.withairbnb.com/t5/Hosting/Guest-is-able-to-block-my-calendar-for-two-months-withou...