I am not sure what you mean by proof of payment. Did you and they have a booking confirmation. If not - there was no valid booking.
What a shame you left it until after these people stayed to check on whether it was a valid booking. You could have contacted Airbnb before letting the guests stay.
Contact numbers for Airbnb are easy to find via Google, your book confirmation gives you ways to contact Airbnb and the 2nd post in the Community Help forum here lists all the ways of contacting them.
I hope it works out for you.