Add a block
On Schedule → Calendar, use Block time, choose the date and time range, and save. You can add a reason for yourself.
What customers see
Nothing. Blocked time simply is not offered as an available slot, and your reason is never shown publicly.
Existing appointments
Blocking time does not cancel anything already booked. If something is booked in that window, move or cancel it yourself.
Block versus hours
A block is an exception — a meeting, an appointment of your own, a delivery. If it happens every week, change your working hours instead.