Each time a rental request is made by a surfer the board host gets a message to approve or decline the rental for the period requested. When the rental gets declined then the surfer's credit card does not get charged. It is up to the board host to decide if the board is available or not.
In the event the board is unavailable or you won't be able to respond to any surfer's rental request it is always possible to unlist the rental. Unlisting a rental is described here: Unlist my rental