A no-show is an empty hour you can no longer sell. You cannot eliminate them completely, but with four measures you can bring the number down sharply.
A day in advance works better than a week. Put the date, time and salon name in the first sentence, so the message can already be read in the notification.
That sounds contradictory, but a customer who can cancel easily does so in good time. Then you can still sell the hour.
For treatments of two hours or more, a deposit via iDEAL is reasonable. Customers understand that, and it cuts cancellations straight away.
Let customers who want an earlier slot join the waitlist. If you cancel an appointment in your calendar, they are notified automatically and the gap does not stay empty.
Cancel within the trial and you pay nothing.