If you have a problem client, for example, someone who frequently cancels or no-shows appointments, there are a few things you can do.
The first thing is to write a note on the client's record, making sure to tick the box to show when they book. Now if you try and make a booking for the client you will be notified of what course of action to take.
Secondly, you can enable deposits on online bookings. This will discourage people from cancelling as they have already paid some or all of the cost of the service. You can see how to enable deposits here: Setting up Stripe Payments
Finally, you can ban specific clients from booking online. To do this tick the box Block from booking online on the client details record. If a client tries to book it won't tell them they are banned, they just won't have any times available to book. This also requires them to have logged into an account when booking.