According to AirBnB Help, to cancel pre-approval:
Go to your Inbox on airbnb.com
Open the message thread with the guest
Click Remove Pre-approval
Only there is nothing like that on the thread. Does anyone know how?
I called AirBnB regarding this and apparently the Help topic is not current. There is no way to remove pre-approval. However, according to the fellow I talked to, pre-approval is automatically cancelled after 24 hours.