Hello @Phil106,
I currently have a student staying with me for a few months. He was given my information by the college that he's currently studying at so his inquiry didn't come through Airbnb.
In this case I just blocked the dates on my calendar for which he'll be here.
I don't know if your situation is like that or whether you recieived the query through Airbnb but the person wants to stay and pay without making the reservation through Airbnb.
In the past, when a guest stayed and then wished to make a return booking I've always required them to make the booking through Airbnb. I also offered a special lower rate.
Steve.