The great thing with Airbnb is that is possible to screen guests. There are different scenarios which can be frustrating though. New member, no reviews (prevents instant booking for example depending on settings), have issues with verification or as we have experienced the credit card was declined on the first go.
The only answer to guests you don't feel comfortable hosting is to decline. If there is a situation, and it has happened, where the enquiry feels extremely out of bounds just report the potential guest to Airbnb.
We have had a couple instances where we have rang support and *poof* - issue solved. I have to admit I do appreciate the host support from Airbnb. Even though we are the product, the company is not afraid to support and delivers exceptionally well for everyone that opens their home to guests.
Most of the time, we do as Queenie and Ted do - help potential guests to a point but never let them dictate terms to us 🙂