The guest block feature has always been hidden a bit. I don’t quite know why...
I personally wish it was possible to manually add people to the block list. I have seen some horrific photos of damage caused by guests on the Blacklist, yet I can’t add these people to my block list. People can make new accounts to avoid this, I know, but I’d like a record of known bad guests in my own settings somewhere.
I feel sorry for the people in Chicago at the moment, with the huge ring of party sellers booking people’s homes and selling tickets... the Blacklist team almost can’t keep up.
Another option for a Instant Book hosts is to give a guest a “thumbs down”. This prevents the guest from Instant booking with you and some other hosts (ones that require positive reviews).