Welcome to hosting @Rob187. I see that you have Instant Booking turned on for your listing. Go to manage listing, booking, and turn it off by requiring travelers to contact you for approval for reservation. I think when setting up a new listing the IB is automatically activated. Until you get your "sea legs", it is best to review guest before they book. Make sense?
I agree with @David126 about cancelling the unexpected guest. Do not cancell yourself. There are several automatic penalties, including blocked dates and financial penalties for host cancelling. Big no-no on Air BNB. You must contact Air BNB so a rep can do the cancellation for you. Here is guide to contacting Air BNB
https://community.withairbnb.com/t5/Help/Contact-Airbnb-A-Community-Help-Guide/m-p/16165#M728
Good luck!!