If you want to block a time slot in the calendar during which an employee should not be bookable, create an internal appointment. Unlike a break, these count as working time in work time statistics and are suitable for, for example, internal tasks and meetings.
Create internal appointment
To block an employee, click on the start time on the desired day and drag an appointment to the desired length. Then enter the purpose of the blocking in the Internal Appointment tab of the appointment dialog.
To enter an internal appointment for multiple employees, drag the appointment across multiple columns.
Short video on entering internal appointments: