If you turn on this setting:
settings > offing control
then your online reservations will auto-cancel if the customer does not make payment in the time-frame you indicate. 30 minutes in this example.
The default is auto-cancel = OFF which is safer.
It will mean you will still see online reservations in your dashboard - but they will simply remain in provisional status if not confirmed with payment (or leaving a credit card).
This will mean if the customer doesn't enter a valid credit card and make payment in that time the reservation will get cancelled. So please use with caution. We recommend 30 minutes as the shortest duration to allow people to find wallet and enter details etc.