When a guest makes a booking, their reservation is confirmed immediately. So there's no way to reject a reservation.
Depending on your policies, a guest might be able to cancel their reservation using a link in their confirmation email or through their Booking account. In some cases, you’re able to initiate a cancellation request if a guest asks you to do so, or if there are issues with guest payments. See below for more info.
Request by guest:
Invalid credit card:
For information on what to do when the guest has an invalid credit card, read this article.
No deposit/pre-payment received:
If you haven't received these as expected, you can find out more about the actions you need to take in this article.