ajaj @Felicity25 you shouldn't accept it before reading his reviews or exchanging a few messages with him. It is not relevant if you respond within 5 minutes or 10 hours, you should give yourself some time..... or turn on instant booking.
If you had the instant booking then you would ill be allowed to call Airbnb and ask them to cancel this booking because your guest have bad reviews and you feel uncomfortable hosting him. But in this case, when you already accepted his request, this is not possible without being penalized.
But I would advise you to try anyway, you have nothing to lose, as you are new to Airbnb maybe you will get some sympathetic rep who will cancel it for you without any penalties.
But if not, then think what would cost you more - trashed home or 50$ fee and super host status? I think you know the answer