A client doesn't turn up
First, check they were actually told
Open the booking, then Email history in its three-dot menu, before you assume anything. If the confirmation was not delivered, they may never have known. See When an email doesn't arrive.
Accusing someone of missing a session they were never told about is a bad conversation, and a surprisingly common one.
Write it down where you'll find it again
There is no no-show button, and a booking has no notes field. Put what happened in the contact's notes instead. Open their record from the booking, then add a note under Contact notes on the Overview tab. Only you can see it.
Write the date, what you did to reach them, and what you agreed afterwards. That note is what you will want months later if a deposit is ever argued over.
Money
Recording a no-show charges and refunds nothing.
What should happen comes from your cancellation policy. That is why having one written into the service or the contract matters. Without it you are negotiating from nothing, in a conversation where one side is already embarrassed.
A deposit already taken is usually kept, if your policy says so.
An unpaid balance is a judgement call. Chasing the full fee for a session that did not happen is hard to argue in law and costly for your business. Many photographers keep the deposit and let the rest go.
If it was your fault, refund promptly and completely. It is the cheapest possible version of that mistake.
Then close the booking
Once the money side is settled, get it off your list.
If the booking was confirmed, use Mark as complete in the three-dot menu. It is only offered on a confirmed booking, and it warns that it "closes the booking and stops reminders and follow-ups. Payments, documents, and calendar history won't be affected."
If it never got that far and was still waiting on a payment or a signature, use Cancel booking instead. Leave Notify contact by email off unless you do want them emailed about it.
If it expired on its own waiting for payment, there is nothing to close. An expired booking is already finished, and Create replacement booking is there if they come back.
Reducing them
Turn on booking reminders. This is the single most effective thing you can do, and they start switched off. Under Settings → Reminders, Booking reminders offers 7 days before, 3 days before, 1 day before and Day of booking. Switching it on ticks the last two for you. Most no-shows are forgetfulness rather than intent. See Reminders.
Take a deposit. People turn up to things they have paid for. On a service, the Booking flow tab's step 4 Payment has Collect payment, which takes it as part of booking.
Put the details where they can find them. Where, when, what to bring. Their booking page carries the date, the location and any meeting link, so send them that link rather than retyping it. A client who cannot find the studio is a no-show in your records and a lost customer in reality.