@Sonya49 Unfortunately, there is nothing you can do about this. Guests have 24 hours after booking to complete payment. Even worse is that they can cancel penalty-free within 48 hours, so there is the potential for them to tie up your calendar for that long.
No, you don't want to cancel, ever, unless it's unavoidable and in that case you'll want to contact Airbnb to tell them why you're not comfortable with the booking, and hopefully it's a reason Airbnb accepts and doesn't penalize you. If you cancel a reservation they don't agreee to do as a "neutral" cancellation, you'll lose your Superhost status and won't be able to be eligible for it again for a year.
Your guest does have a legitimate concern. I had a guest who has used Airbnb a lot and at some point her account was hacked and false charges made to her credit card. She told me that now, although she does pay for her bookings with a credit card, as soon as her stay is over and the reviews have been submitted, she pulls her credit card info from the site until the next time she wants to book.