Taking a slot back

An open slot is yours to cancel; a booked one is an agreement with a buyer, and has to be resolved with them.

Each open slot has a Cancel slot button. Cancelling removes it from the listing so nobody can book it.

A slot that someone has already booked is a different matter: the booking is an agreement with a buyer, and it has to be resolved with them rather than removed from underneath them. Cancel the booking itself, and talk to the buyer first if it is already paid.