During the four years that my mom and I have been hosting there has only been one guest that we have wished to block. Another host has asked me today how to do the same for a bad guest they have had. I have never found any option to do this from my mobile app or laptop. How can hosts block unwanted guests?
Have a look at the Airbnb Help website for full instructions but basically you can into the messaging with the guest and flag why you want to do so @Andrea8411
Or tick the would not host again feature on the guest review .
Hope you are having a great start of the week. Following Helen's answer, I'm attaching a link from the Help Center that might help to answer your question.